diff --git a/build-logic/build-parameters/build.gradle.kts b/build-logic/build-parameters/build.gradle.kts
new file mode 100644
index 0000000000..90490ce5b1
--- /dev/null
+++ b/build-logic/build-parameters/build.gradle.kts
@@ -0,0 +1,15 @@
+plugins {
+    id("org.gradlex.build-parameters") version "1.4.3"
+}
+
+buildParameters {
+    pluginId("testng.build-parameters")
+    bool("enableCheckerframework") {
+        defaultValue.set(false)
+        description.set("Run CheckerFramework (nullness) verifications")
+    }
+    bool("enableErrorprone") {
+        defaultValue.set(true)
+        description.set("Enable ErrorProne verifications")
+    }
+}
diff --git a/build-logic/code-quality/build.gradle.kts b/build-logic/code-quality/build.gradle.kts
index a83857d4ef..76030e00c5 100644
--- a/build-logic/code-quality/build.gradle.kts
+++ b/build-logic/code-quality/build.gradle.kts
@@ -11,6 +11,8 @@ repositories {
 dependencies {
     implementation("org.sonarqube:org.sonarqube.gradle.plugin:2.8")
     implementation("com.github.autostyle:autostyle-plugin-gradle:3.1")
+    implementation("org.checkerframework:checkerframework-gradle-plugin:0.6.29")
+    implementation("net.ltgt.gradle:gradle-errorprone-plugin:3.1.0")
 }
 
 tasks.withType<KotlinCompile>().configureEach {
diff --git a/build-logic/code-quality/src/main/kotlin/testng.checkerframework.gradle.kts b/build-logic/code-quality/src/main/kotlin/testng.checkerframework.gradle.kts
new file mode 100644
index 0000000000..f9a1994e19
--- /dev/null
+++ b/build-logic/code-quality/src/main/kotlin/testng.checkerframework.gradle.kts
@@ -0,0 +1,25 @@
+import org.checkerframework.gradle.plugin.CheckerFrameworkExtension
+
+plugins {
+    id("org.checkerframework")
+}
+
+dependencies {
+    checkerFramework("org.checkerframework:checker:3.36.0")
+}
+
+configure<CheckerFrameworkExtension> {
+    checkers = listOf(
+        "org.checkerframework.checker.nullness.NullnessChecker",
+        "org.checkerframework.checker.optional.OptionalChecker"
+    )
+}
+
+tasks.withType<JavaCompile>().configureEach {
+    // Don't run the checker on generated code.
+    if (name.startsWith("compileMainGenerated")) {
+        checkerFramework {
+            skipCheckerFramework = true
+        }
+    }
+}
diff --git a/build-logic/code-quality/src/main/kotlin/testng.errorprone.gradle.kts b/build-logic/code-quality/src/main/kotlin/testng.errorprone.gradle.kts
new file mode 100644
index 0000000000..16c178029c
--- /dev/null
+++ b/build-logic/code-quality/src/main/kotlin/testng.errorprone.gradle.kts
@@ -0,0 +1,13 @@
+import net.ltgt.gradle.errorprone.errorprone
+
+plugins {
+    id("net.ltgt.errorprone")
+}
+
+dependencies {
+    errorprone("com.google.errorprone:error_prone_core:2.20.0")
+}
+
+tasks.withType<JavaCompile>().configureEach {
+    options.errorprone.disableWarningsInGeneratedCode.set(true)
+}
diff --git a/build-logic/jvm/src/main/kotlin/testng.java.gradle.kts b/build-logic/jvm/src/main/kotlin/testng.java.gradle.kts
index a2717c9f93..b8f3586f41 100644
--- a/build-logic/jvm/src/main/kotlin/testng.java.gradle.kts
+++ b/build-logic/jvm/src/main/kotlin/testng.java.gradle.kts
@@ -2,6 +2,7 @@ plugins {
     `java-base`
     id("testng.versioning")
     id("testng.style")
+    // id("testng.build-parameters") // Plugin [id: 'testng.build-parameters'] was not found
     id("testng.repositories")
     // Improves Gradle Test logging
     // See https://github.com/vlsi/vlsi-release-plugins/tree/master/plugins/gradle-extensions-plugin
@@ -13,6 +14,13 @@ java {
     targetCompatibility = JavaVersion.VERSION_11
 }
 
+if (true /*buildParameters.enableCheckerframework*/) {
+    apply(plugin = "testng.checkerframework")
+}
+if (true /*buildParameters.enableErrorprone*/) {
+    apply(plugin = "testng.errorprone")
+}
+
 tasks.withType<JavaCompile>().configureEach {
     inputs.property("java.version", System.getProperty("java.version"))
     inputs.property("java.vendor", System.getProperty("java.vendor"))
diff --git a/build-logic/settings.gradle.kts b/build-logic/settings.gradle.kts
index ee7e4ab97b..b4e114e7c5 100644
--- a/build-logic/settings.gradle.kts
+++ b/build-logic/settings.gradle.kts
@@ -7,6 +7,7 @@ dependencyResolutionManagement {
 rootProject.name = "build-logic"
 
 include(":basics")
+include(":build-parameters")
 include(":code-quality")
 include(":jvm")
 include(":publishing")
diff --git a/testng-asserts/src/main/java/org/testng/Assert.java b/testng-asserts/src/main/java/org/testng/Assert.java
index 996196c31a..2502a81cef 100644
--- a/testng-asserts/src/main/java/org/testng/Assert.java
+++ b/testng-asserts/src/main/java/org/testng/Assert.java
@@ -17,6 +17,7 @@
 import java.util.function.Predicate;
 import java.util.stream.Collectors;
 import java.util.stream.StreamSupport;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.collections.Lists;
 
 /**
@@ -52,7 +53,7 @@ protected Assert() {
    * @param condition the condition to evaluate
    * @param message the assertion error message
    */
-  public static void assertTrue(boolean condition, String message) {
+  public static void assertTrue(boolean condition, @Nullable String message) {
     if (!condition) {
       failNotEquals(condition, Boolean.TRUE, message);
     }
@@ -74,7 +75,7 @@ public static void assertTrue(boolean condition) {
    * @param condition the condition to evaluate
    * @param message the assertion error message
    */
-  public static void assertFalse(boolean condition, String message) {
+  public static void assertFalse(boolean condition, @Nullable String message) {
     if (condition) {
       failNotEquals(condition, Boolean.FALSE, message); // TESTNG-81
     }
@@ -107,7 +108,8 @@ public static void fail(String message, Throwable realCause) {
    *
    * @param message the assertion error message
    */
-  public static void fail(String message) {
+  // See https://github.com/typetools/checker-framework/issues/2076
+  public static /* @ThrowsException */ void fail(@Nullable String message) {
     throw new AssertionError(message);
   }
 
@@ -124,7 +126,7 @@ public static void fail() {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Object actual, Object expected, String message) {
+  public static void assertEquals(Object actual, Object expected, @Nullable String message) {
     if (expected != null && expected.getClass().isArray()) {
       assertArrayEquals(actual, expected, message);
       return;
@@ -144,21 +146,23 @@ private static boolean areEqual(Object actual, Object expected) {
    * consideration hence comparing them by reference. Intended to be called directly to test
    * equality of collections content.
    */
-  private static void assertEqualsImpl(Object actual, Object expected, String message) {
+  private static void assertEqualsImpl(
+      @Nullable Object actual, @Nullable Object expected, @Nullable String message) {
     boolean equal = areEqualImpl(actual, expected);
     if (!equal) {
       failNotEquals(actual, expected, message);
     }
   }
 
-  private static void assertNotEqualsImpl(Object actual, Object expected, String message) {
+  private static void assertNotEqualsImpl(
+      @Nullable Object actual, @Nullable Object expected, @Nullable String message) {
     boolean notEqual = areNotEqualImpl(actual, expected);
     if (!notEqual) {
       failEquals(actual, expected, message);
     }
   }
 
-  private static boolean areNotEqualImpl(Object actual, Object expected) {
+  private static boolean areNotEqualImpl(@Nullable Object actual, @Nullable Object expected) {
     if (expected == null) {
       return actual != null;
     }
@@ -169,7 +173,7 @@ private static boolean areNotEqualImpl(Object actual, Object expected) {
     return !expected.equals(actual);
   }
 
-  private static boolean areEqualImpl(Object actual, Object expected) {
+  private static boolean areEqualImpl(@Nullable Object actual, @Nullable Object expected) {
     if ((expected == null) && (actual == null)) {
       return true;
     }
@@ -181,7 +185,8 @@ private static boolean areEqualImpl(Object actual, Object expected) {
   }
 
   /** returns not equal reason or null if equal */
-  private static String getArrayNotEqualReason(Object actual, Object expected) {
+  private static @Nullable String getArrayNotEqualReason(
+      @Nullable Object actual, @Nullable Object expected) {
     if (Objects.equals(actual, expected)) {
       return null;
     }
@@ -212,15 +217,17 @@ private static boolean areArraysEqual(Object actual, Object expected) {
     return getArrayNotEqualReason(actual, expected) == null;
   }
 
-  private static void assertArrayEquals(Object actual, Object expected, String message) {
-    String reason = getArrayNotEqualReason(actual, expected);
+  private static void assertArrayEquals(
+      @Nullable Object actual, @Nullable Object expected, @Nullable String message) {
+    @Nullable String reason = getArrayNotEqualReason(actual, expected);
     if (null != reason) {
       failNotEquals(actual, expected, message == null ? "" : message + " (" + message + ")");
     }
   }
 
-  private static void assertArrayNotEquals(Object actual, Object expected, String message) {
-    String reason = getArrayNotEqualReason(actual, expected);
+  private static void assertArrayNotEquals(
+      @Nullable Object actual, @Nullable Object expected, @Nullable String message) {
+    @Nullable String reason = getArrayNotEqualReason(actual, expected);
     if (null == reason) {
       failEquals(actual, expected, message);
     }
@@ -652,7 +659,7 @@ public static void assertEquals(Object actual, Object expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(String actual, String expected, String message) {
+  public static void assertEquals(String actual, String expected, @Nullable String message) {
     assertEquals((Object) actual, (Object) expected, message);
   }
 
@@ -693,7 +700,8 @@ private static boolean areEqual(double actual, double expected, double delta) {
    * @param delta the absolute tolerable difference between the actual and expected values
    * @param message the assertion error message
    */
-  public static void assertEquals(double actual, double expected, double delta, String message) {
+  public static void assertEquals(
+      double actual, double expected, double delta, @Nullable String message) {
     if (!areEqual(actual, expected, delta)) {
       failNotEquals(actual, expected, message);
     }
@@ -719,7 +727,7 @@ public static void assertEquals(double actual, double expected, double delta) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(double actual, double expected, String message) {
+  public static void assertEquals(double actual, double expected, @Nullable String message) {
     if (Double.isNaN(expected)) {
       if (!Double.isNaN(actual)) {
         failNotEquals(actual, expected, message);
@@ -761,7 +769,7 @@ public static void assertEquals(double actual, Double expected, String message)
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Double actual, Double expected, String message) {
+  public static void assertEquals(Double actual, Double expected, @Nullable String message) {
     assertEquals(actual, (Object) expected, message);
   }
 
@@ -832,7 +840,8 @@ private static boolean areEqual(float actual, float expected, float delta) {
    * @param delta the absolute tolerable difference between the actual and expected values
    * @param message the assertion error message
    */
-  public static void assertEquals(float actual, float expected, float delta, String message) {
+  public static void assertEquals(
+      float actual, float expected, float delta, @Nullable String message) {
     if (!areEqual(actual, expected, delta)) {
       failNotEquals(actual, expected, message);
     }
@@ -858,7 +867,7 @@ public static void assertEquals(float actual, float expected, float delta) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(float actual, float expected, String message) {
+  public static void assertEquals(float actual, float expected, @Nullable String message) {
     if (Float.isNaN(expected)) {
       if (!Float.isNaN(actual)) {
         failNotEquals(actual, expected, message);
@@ -900,7 +909,7 @@ public static void assertEquals(float actual, Float expected, String message) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Float actual, Float expected, String message) {
+  public static void assertEquals(Float actual, Float expected, @Nullable String message) {
     assertEquals(actual, (Object) expected, message);
   }
 
@@ -952,7 +961,7 @@ public static void assertEquals(Float actual, Float expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(long actual, long expected, String message) {
+  public static void assertEquals(long actual, long expected, @Nullable String message) {
     assertEquals(Long.valueOf(actual), Long.valueOf(expected), message);
   }
 
@@ -976,7 +985,7 @@ public static void assertEquals(Long actual, long expected, String message) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Long actual, Long expected, String message) {
+  public static void assertEquals(Long actual, Long expected, @Nullable String message) {
     assertEquals(actual, (Object) expected, message);
   }
 
@@ -1027,7 +1036,7 @@ public static void assertEquals(Long actual, Long expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(boolean actual, boolean expected, String message) {
+  public static void assertEquals(boolean actual, boolean expected, @Nullable String message) {
     assertEquals(Boolean.valueOf(actual), Boolean.valueOf(expected), message);
   }
 
@@ -1063,7 +1072,7 @@ public static void assertEquals(boolean actual, Boolean expected, String message
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Boolean actual, Boolean expected, String message) {
+  public static void assertEquals(Boolean actual, Boolean expected, @Nullable String message) {
     assertEquals(actual, (Object) expected, message);
   }
 
@@ -1115,7 +1124,7 @@ public static void assertEquals(boolean actual, Boolean expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(byte actual, byte expected, String message) {
+  public static void assertEquals(byte actual, byte expected, @Nullable String message) {
     assertEquals(Byte.valueOf(actual), Byte.valueOf(expected), message);
   }
 
@@ -1151,7 +1160,7 @@ public static void assertEquals(byte actual, Byte expected, String message) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Byte actual, Byte expected, String message) {
+  public static void assertEquals(Byte actual, Byte expected, @Nullable String message) {
     assertEquals(actual, (Object) expected, message);
   }
 
@@ -1203,7 +1212,7 @@ public static void assertEquals(byte actual, Byte expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(char actual, char expected, String message) {
+  public static void assertEquals(char actual, char expected, @Nullable String message) {
     assertEquals(Character.valueOf(actual), Character.valueOf(expected), message);
   }
 
@@ -1239,7 +1248,7 @@ public static void assertEquals(char actual, Character expected, String message)
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Character actual, Character expected, String message) {
+  public static void assertEquals(Character actual, Character expected, @Nullable String message) {
     assertEquals(actual, (Object) expected, message);
   }
 
@@ -1291,7 +1300,7 @@ public static void assertEquals(Character actual, Character expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(short actual, short expected, String message) {
+  public static void assertEquals(short actual, short expected, @Nullable String message) {
     assertEquals(Short.valueOf(actual), Short.valueOf(expected), message);
   }
 
@@ -1327,7 +1336,7 @@ public static void assertEquals(short actual, Short expected, String message) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Short actual, Short expected, String message) {
+  public static void assertEquals(Short actual, Short expected, @Nullable String message) {
     assertEquals(actual, (Object) expected, message);
   }
 
@@ -1379,7 +1388,7 @@ public static void assertEquals(Short actual, Short expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(int actual, int expected, String message) {
+  public static void assertEquals(int actual, int expected, @Nullable String message) {
     assertEquals(Integer.valueOf(actual), Integer.valueOf(expected), message);
   }
 
@@ -1415,7 +1424,7 @@ public static void assertEquals(int actual, Integer expected, String message) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Integer actual, Integer expected, String message) {
+  public static void assertEquals(Integer actual, Integer expected, @Nullable String message) {
     assertEquals(actual, (Object) expected, message);
   }
 
@@ -1475,7 +1484,7 @@ public static void assertNotNull(Object object) {
    * @param object the assertion object
    * @param message the assertion error message
    */
-  public static void assertNotNull(Object object, String message) {
+  public static void assertNotNull(Object object, @Nullable String message) {
     if (object == null) {
       String formatted = "";
       if (message != null) {
@@ -1502,7 +1511,7 @@ public static void assertNull(Object object) {
    * @param object the assertion object
    * @param message the assertion error message
    */
-  public static void assertNull(Object object, String message) {
+  public static void assertNull(@Nullable Object object, @Nullable String message) {
     if (object != null) {
       failNotSame(object, null, message);
     }
@@ -1516,7 +1525,7 @@ public static void assertNull(Object object, String message) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertSame(Object actual, Object expected, String message) {
+  public static void assertSame(Object actual, Object expected, @Nullable String message) {
     if (expected == actual) {
       return;
     }
@@ -1541,7 +1550,7 @@ public static void assertSame(Object actual, Object expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertNotSame(Object actual, Object expected, String message) {
+  public static void assertNotSame(Object actual, Object expected, @Nullable String message) {
     if (expected == actual) {
       failSame(actual, expected, message);
     }
@@ -1558,7 +1567,7 @@ public static void assertNotSame(Object actual, Object expected) {
     assertNotSame(actual, expected, null);
   }
 
-  private static void failSame(Object actual, Object expected, String message) {
+  private static void failSame(Object actual, Object expected, @Nullable String message) {
     String formatted = "";
     if (message != null) {
       formatted = message + " ";
@@ -1566,7 +1575,8 @@ private static void failSame(Object actual, Object expected, String message) {
     fail(formatted + ASSERT_LEFT2 + expected + ASSERT_MIDDLE + actual + ASSERT_RIGHT);
   }
 
-  private static void failNotSame(Object actual, Object expected, String message) {
+  private static void failNotSame(
+      @Nullable Object actual, @Nullable Object expected, @Nullable String message) {
     String formatted = "";
     if (message != null) {
       formatted = message + " ";
@@ -1574,15 +1584,21 @@ private static void failNotSame(Object actual, Object expected, String message)
     fail(formatted + ASSERT_EQUAL_LEFT + expected + ASSERT_MIDDLE + actual + ASSERT_RIGHT);
   }
 
-  private static void failNotEquals(Object actual, Object expected, String message) {
+  private static void failNotEquals(
+      @Nullable Object actual, @Nullable Object expected, @Nullable String message) {
     fail(format(actual, expected, message, true));
   }
 
-  private static void failEquals(Object actual, Object expected, String message) {
+  private static void failEquals(
+      @Nullable Object actual, @Nullable Object expected, @Nullable String message) {
     fail(format(actual, expected, message, false));
   }
 
-  static String format(Object actual, Object expected, String message, boolean isAssertEquals) {
+  static String format(
+      @Nullable Object actual,
+      @Nullable Object expected,
+      @Nullable String message,
+      boolean isAssertEquals) {
     String formatted = "";
     if (null != message) {
       formatted = message + " ";
@@ -1602,7 +1618,8 @@ static String format(Object actual, Object expected, String message, boolean isA
    * @param actual the actual value
    * @param expected the expected value
    */
-  public static void assertEquals(Collection<?> actual, Collection<?> expected) {
+  public static void assertEquals(
+      @Nullable Collection<@Nullable ?> actual, @Nullable Collection<@Nullable ?> expected) {
     assertEquals(actual, expected, null);
   }
 
@@ -1614,7 +1631,10 @@ public static void assertEquals(Collection<?> actual, Collection<?> expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Collection<?> actual, Collection<?> expected, String message) {
+  public static void assertEquals(
+      @Nullable Collection<@Nullable ?> actual,
+      @Nullable Collection<@Nullable ?> expected,
+      @Nullable String message) {
     if (actual == expected) { // We don't use Objects.equals here because order is checked
       return;
     }
@@ -1625,6 +1645,7 @@ public static void assertEquals(Collection<?> actual, Collection<?> expected, St
       } else {
         fail("Collections not equal: expected: " + expected + " and actual: " + actual);
       }
+      return;
     }
 
     assertEquals(
@@ -1632,13 +1653,13 @@ public static void assertEquals(Collection<?> actual, Collection<?> expected, St
         expected.size(),
         (message == null ? "" : message + ": ") + "lists don't have the same size");
 
-    Iterator<?> actIt = actual.iterator();
-    Iterator<?> expIt = expected.iterator();
+    Iterator<@Nullable ?> actIt = actual.iterator();
+    Iterator<@Nullable ?> expIt = expected.iterator();
     int i = -1;
     while (actIt.hasNext() && expIt.hasNext()) {
       i++;
-      Object e = expIt.next();
-      Object a = actIt.next();
+      @Nullable Object e = expIt.next();
+      @Nullable Object a = actIt.next();
       String explanation = "Lists differ at element [" + i + "]: " + e + " != " + a;
       String errorMessage = message == null ? explanation : message + ": " + explanation;
       assertEqualsImpl(a, e, errorMessage);
@@ -1653,7 +1674,7 @@ public static void assertEquals(Collection<?> actual, Collection<?> expected, St
    * @param actual the actual value
    * @param expected the expected value
    */
-  public static void assertEquals(Iterator<?> actual, Iterator<?> expected) {
+  public static void assertEquals(@Nullable Iterator<?> actual, @Nullable Iterator<?> expected) {
     assertEquals(actual, expected, null);
   }
 
@@ -1666,7 +1687,10 @@ public static void assertEquals(Iterator<?> actual, Iterator<?> expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Iterator<?> actual, Iterator<?> expected, String message) {
+  public static void assertEquals(
+      @Nullable Iterator<@Nullable ?> actual,
+      @Nullable Iterator<@Nullable ?> expected,
+      @Nullable String message) {
     if (actual == expected) { // We don't use Objects.equals here because order is checked
       return;
     }
@@ -1676,14 +1700,15 @@ public static void assertEquals(Iterator<?> actual, Iterator<?> expected, String
               ? message
               : "Iterators not equal: expected: " + expected + " and actual: " + actual;
       fail(msg);
+      return;
     }
 
     int i = -1;
     while (actual.hasNext() && expected.hasNext()) {
 
       i++;
-      Object e = expected.next();
-      Object a = actual.next();
+      @Nullable Object e = expected.next();
+      @Nullable Object a = actual.next();
       String explanation = "Iterators differ at element [" + i + "]: " + e + " != " + a;
       String errorMessage = message == null ? explanation : message + ": " + explanation;
 
@@ -1711,7 +1736,8 @@ public static void assertEquals(Iterator<?> actual, Iterator<?> expected, String
    * @param actual the actual value
    * @param expected the expected value
    */
-  public static void assertEquals(Iterable<?> actual, Iterable<?> expected) {
+  public static void assertEquals(
+      @Nullable Iterable<@Nullable ?> actual, @Nullable Iterable<@Nullable ?> expected) {
     assertEquals(actual, expected, null);
   }
 
@@ -1723,7 +1749,10 @@ public static void assertEquals(Iterable<?> actual, Iterable<?> expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Iterable<?> actual, Iterable<?> expected, String message) {
+  public static void assertEquals(
+      @Nullable Iterable<@Nullable ?> actual,
+      @Nullable Iterable<@Nullable ?> expected,
+      @Nullable String message) {
     if (actual == expected) { // We don't use Objects.equals here because order is checked
       return;
     }
@@ -1734,10 +1763,11 @@ public static void assertEquals(Iterable<?> actual, Iterable<?> expected, String
       } else {
         fail("Iterables not equal: expected: " + expected + " and actual: " + actual);
       }
+      return;
     }
 
-    Iterator<?> actIt = actual.iterator();
-    Iterator<?> expIt = expected.iterator();
+    Iterator<@Nullable ?> actIt = actual.iterator();
+    Iterator<@Nullable ?> expIt = expected.iterator();
 
     assertEquals(actIt, expIt, message);
   }
@@ -1750,12 +1780,15 @@ public static void assertEquals(Iterable<?> actual, Iterable<?> expected, String
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEquals(Object[] actual, Object[] expected, String message) {
+  public static void assertEquals(
+      @Nullable Object @Nullable [] actual,
+      @Nullable Object @Nullable [] expected,
+      @Nullable String message) {
     if (Arrays.equals(actual, expected)) {
       return;
     }
 
-    if ((actual == null && expected != null) || (actual != null && expected == null)) {
+    if (actual == null || expected == null) {
       if (message != null) {
         fail(message);
       } else {
@@ -1765,6 +1798,7 @@ public static void assertEquals(Object[] actual, Object[] expected, String messa
                 + " and actual: "
                 + Arrays.toString(actual));
       }
+      return;
     }
     if (actual.length != expected.length) {
       failAssertNoEqual(
@@ -1772,14 +1806,14 @@ public static void assertEquals(Object[] actual, Object[] expected, String messa
     }
 
     for (int i = 0; i < expected.length; i++) {
-      Object e = expected[i];
-      Object a = actual[i];
+      @Nullable Object e = expected[i];
+      @Nullable Object a = actual[i];
       String explanation = "Arrays differ at element [" + i + "]: " + e + " != " + a;
       String errorMessage = message == null ? explanation : message + ": " + explanation;
       if (a == null && e == null) {
         continue;
       }
-      if ((a == null && e != null) || (a != null && e == null)) {
+      if (a == null || e == null) {
         failNotEquals(a, e, message);
       }
       // Compare by value for multi-dimensional array.
@@ -1801,7 +1835,10 @@ public static void assertEquals(Object[] actual, Object[] expected, String messa
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEqualsNoOrder(Object[] actual, Object[] expected, String message) {
+  public static void assertEqualsNoOrder(
+      @Nullable Object @Nullable [] actual,
+      @Nullable Object @Nullable [] expected,
+      @Nullable String message) {
     if (actual == expected) { // We don't use Arrays.equals here because order is not checked
       return;
     }
@@ -1813,14 +1850,16 @@ public static void assertEqualsNoOrder(Object[] actual, Object[] expected, Strin
               + " and actual: "
               + Arrays.toString(actual),
           message);
+      return;
     }
 
     if (actual.length != expected.length) {
       failAssertNoEqual(
           "Arrays do not have the same size:" + actual.length + " != " + expected.length, message);
+      return;
     }
 
-    List<Object> actualCollection = Lists.newArrayList(actual);
+    List<@Nullable Object> actualCollection = Lists.newArrayList(actual);
     for (Object o : expected) {
       actualCollection.remove(o);
     }
@@ -1843,11 +1882,12 @@ public static void assertEqualsNoOrder(Object[] actual, Object[] expected, Strin
    * @param message the assertion error message
    */
   public static void assertEqualsNoOrder(
-      Collection<?> actual, Collection<?> expected, String message) {
+      Collection<@Nullable ?> actual, Collection<@Nullable ?> expected, @Nullable String message) {
     if (actual.size() != expected.size()) {
       failAssertNoEqual(
           "Collections do not have the same size: " + actual.size() + " != " + expected.size(),
           message);
+      return;
     }
 
     List<?> actualCollection = Lists.newArrayList(actual);
@@ -1866,7 +1906,8 @@ public static void assertEqualsNoOrder(
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertEqualsNoOrder(Iterator<?> actual, Iterator<?> expected, String message) {
+  public static void assertEqualsNoOrder(
+      Iterator<@Nullable ?> actual, Iterator<@Nullable ?> expected, @Nullable String message) {
     List<?> actualCollection = Lists.newArrayList(actual);
     List<?> expectedCollection = Lists.newArrayList(expected);
 
@@ -1877,6 +1918,7 @@ public static void assertEqualsNoOrder(Iterator<?> actual, Iterator<?> expected,
               + " != "
               + expectedCollection.size(),
           message);
+      return;
     }
 
     actualCollection.removeAll(expectedCollection);
@@ -1890,7 +1932,7 @@ public static void assertEqualsNoOrder(Iterator<?> actual, Iterator<?> expected,
     }
   }
 
-  private static String toString(Iterator<?> iterator) {
+  private static @Nullable String toString(@Nullable Iterator<?> iterator) {
     if (iterator == null) {
       return null;
     }
@@ -1900,7 +1942,7 @@ private static String toString(Iterator<?> iterator) {
         .collect(Collectors.joining(", "));
   }
 
-  private static void failAssertNoEqual(String defaultMessage, String message) {
+  private static void failAssertNoEqual(String defaultMessage, @Nullable String message) {
     if (message != null) {
       fail(message);
     } else {
@@ -1915,7 +1957,8 @@ private static void failAssertNoEqual(String defaultMessage, String message) {
    * @param actual the actual value
    * @param expected the expected value
    */
-  public static void assertEquals(Object[] actual, Object[] expected) {
+  public static void assertEquals(
+      @Nullable Object @Nullable [] actual, @Nullable Object @Nullable [] expected) {
     assertEquals(actual, expected, null);
   }
 
@@ -1928,7 +1971,8 @@ public static void assertEquals(Object[] actual, Object[] expected) {
    * @param actual the actual value
    * @param expected the expected value
    */
-  public static void assertEqualsNoOrder(Object[] actual, Object[] expected) {
+  public static void assertEqualsNoOrder(
+      @Nullable Object @Nullable [] actual, @Nullable Object @Nullable [] expected) {
     assertEqualsNoOrder(actual, expected, null);
   }
 
@@ -1939,7 +1983,8 @@ public static void assertEqualsNoOrder(Object[] actual, Object[] expected) {
    * @param actual the actual value
    * @param expected the expected value
    */
-  public static void assertEqualsNoOrder(Collection<?> actual, Collection<?> expected) {
+  public static void assertEqualsNoOrder(
+      Collection<@Nullable ?> actual, Collection<@Nullable ?> expected) {
     assertEqualsNoOrder(actual, expected, null);
   }
 
@@ -1950,7 +1995,8 @@ public static void assertEqualsNoOrder(Collection<?> actual, Collection<?> expec
    * @param actual the actual value
    * @param expected the expected value
    */
-  public static void assertEqualsNoOrder(Iterator<?> actual, Iterator<?> expected) {
+  public static void assertEqualsNoOrder(
+      Iterator<@Nullable ?> actual, Iterator<@Nullable ?> expected) {
     assertEqualsNoOrder(actual, expected, null);
   }
 
@@ -1960,12 +2006,13 @@ public static void assertEqualsNoOrder(Iterator<?> actual, Iterator<?> expected)
    * @param actual The actual value
    * @param expected The expected value
    */
-  public static void assertEquals(Set<?> actual, Set<?> expected) {
+  public static void assertEquals(@Nullable Set<?> actual, @Nullable Set<?> expected) {
     assertEquals(actual, expected, null);
   }
 
   /** returns not equal reason or null if equal */
-  private static String getNotEqualReason(Collection<?> actual, Collection<?> expected) {
+  private static @Nullable String getNotEqualReason(
+      @Nullable Collection<?> actual, @Nullable Collection<?> expected) {
     if (actual == expected) { // We don't use Arrays.equals here because order is checked
       return null;
     }
@@ -1982,7 +2029,8 @@ private static String getNotEqualReason(Collection<?> actual, Collection<?> expe
     return getNotEqualReason(actual.iterator(), expected.iterator());
   }
 
-  private static String getNotEqualReason(Iterator<?> actual, Iterator<?> expected) {
+  private static @Nullable String getNotEqualReason(
+      @Nullable Iterator<@Nullable ?> actual, @Nullable Iterator<@Nullable ?> expected) {
     if (actual == expected) { // We don't use Arrays.equals here because order is checked
       return null;
     }
@@ -2006,7 +2054,8 @@ private static String getNotEqualReason(Iterator<?> actual, Iterator<?> expected
     return null;
   }
 
-  private static String getNotEqualReason(Set<?> actual, Set<?> expected) {
+  private static @Nullable String getNotEqualReason(
+      @Nullable Set<@Nullable ?> actual, @Nullable Set<@Nullable ?> expected) {
     if (actual == expected) {
       return null;
     }
@@ -2029,8 +2078,11 @@ private static String getNotEqualReason(Set<?> actual, Set<?> expected) {
    * @param expected The expected value
    * @param message The message
    */
-  public static void assertEquals(Set<?> actual, Set<?> expected, String message) {
-    String notEqualReason = getNotEqualReason(actual, expected);
+  public static void assertEquals(
+      @Nullable Set<@Nullable ?> actual,
+      @Nullable Set<@Nullable ?> expected,
+      @Nullable String message) {
+    @Nullable String notEqualReason = getNotEqualReason(actual, expected);
     if (null != notEqualReason) {
       // Keep the back compatible
       if (message == null) {
@@ -2042,7 +2094,8 @@ public static void assertEquals(Set<?> actual, Set<?> expected, String message)
   }
 
   /** returns not equal deep reason or null if equal */
-  private static String getNotEqualDeepReason(Set<?> actual, Set<?> expected) {
+  private static @Nullable String getNotEqualDeepReason(
+      @Nullable Set<@Nullable ?> actual, @Nullable Set<@Nullable ?> expected) {
     if (Objects.equals(actual, expected)) {
       return null;
     }
@@ -2062,7 +2115,7 @@ private static String getNotEqualDeepReason(Set<?> actual, Set<?> expected) {
       Object expectedValue = expectedIterator.next();
       Object value = actualIterator.next();
       if (expectedValue.getClass().isArray()) {
-        String arrayNotEqualReason = getArrayNotEqualReason(value, expectedValue);
+        @Nullable String arrayNotEqualReason = getArrayNotEqualReason(value, expectedValue);
         if (arrayNotEqualReason != null) {
           return arrayNotEqualReason;
         }
@@ -2075,8 +2128,11 @@ private static String getNotEqualDeepReason(Set<?> actual, Set<?> expected) {
     return null;
   }
 
-  public static void assertEqualsDeep(Set<?> actual, Set<?> expected, String message) {
-    String notEqualDeepReason = getNotEqualDeepReason(actual, expected);
+  public static void assertEqualsDeep(
+      @Nullable Set<@Nullable ?> actual,
+      @Nullable Set<@Nullable ?> expected,
+      @Nullable String message) {
+    @Nullable String notEqualDeepReason = getNotEqualDeepReason(actual, expected);
     if (notEqualDeepReason != null) {
       if (message == null) {
         fail(notEqualDeepReason);
@@ -2086,11 +2142,12 @@ public static void assertEqualsDeep(Set<?> actual, Set<?> expected, String messa
     }
   }
 
-  public static void assertEquals(Map<?, ?> actual, Map<?, ?> expected) {
+  public static void assertEquals(@Nullable Map<?, ?> actual, @Nullable Map<?, ?> expected) {
     assertEquals(actual, expected, null);
   }
 
-  private static String getNotEqualReason(Map<?, ?> actual, Map<?, ?> expected) {
+  private static @Nullable String getNotEqualReason(
+      @Nullable Map<?, ?> actual, @Nullable Map<?, ?> expected) {
     if (Objects.equals(actual, expected)) {
       return null;
     }
@@ -2128,8 +2185,9 @@ private static String getNotEqualReason(Map<?, ?> actual, Map<?, ?> expected) {
    * @param expected The expected value
    * @param message The message
    */
-  public static void assertEquals(Map<?, ?> actual, Map<?, ?> expected, String message) {
-    String notEqualReason = getNotEqualReason(actual, expected);
+  public static void assertEquals(
+      @Nullable Map<?, ?> actual, @Nullable Map<?, ?> expected, @Nullable String message) {
+    @Nullable String notEqualReason = getNotEqualReason(actual, expected);
     if (notEqualReason != null) {
       if (message == null) {
         fail(notEqualReason);
@@ -2139,12 +2197,13 @@ public static void assertEquals(Map<?, ?> actual, Map<?, ?> expected, String mes
     }
   }
 
-  public static void assertEqualsDeep(Map<?, ?> actual, Map<?, ?> expected) {
+  public static void assertEqualsDeep(@Nullable Map<?, ?> actual, @Nullable Map<?, ?> expected) {
     assertEqualsDeep(actual, expected, null);
   }
 
   /** returns not equal deep reason or null if equal */
-  private static String getNotEqualDeepReason(Map<?, ?> actual, Map<?, ?> expected) {
+  private static @Nullable String getNotEqualDeepReason(
+      @Nullable Map<?, ?> actual, @Nullable Map<?, ?> expected) {
     if (Objects.equals(actual, expected)) {
       return null;
     }
@@ -2178,8 +2237,9 @@ private static String getNotEqualDeepReason(Map<?, ?> actual, Map<?, ?> expected
     return null;
   }
 
-  public static void assertEqualsDeep(Map<?, ?> actual, Map<?, ?> expected, String message) {
-    String notEqualDeepReason = getNotEqualDeepReason(actual, expected);
+  public static void assertEqualsDeep(
+      @Nullable Map<?, ?> actual, @Nullable Map<?, ?> expected, @Nullable String message) {
+    @Nullable String notEqualDeepReason = getNotEqualDeepReason(actual, expected);
     if (notEqualDeepReason != null) {
       if (message == null) {
         fail(notEqualDeepReason);
@@ -2193,7 +2253,8 @@ public static void assertEqualsDeep(Map<?, ?> actual, Map<?, ?> expected, String
   // assertNotEquals
   //
 
-  public static void assertNotEquals(Object actual, Object expected, String message) {
+  public static void assertNotEquals(
+      @Nullable Object actual, @Nullable Object expected, @Nullable String message) {
     if (expected != null && expected.getClass().isArray()) {
       assertArrayNotEquals(actual, expected, message);
       return;
diff --git a/testng-asserts/src/main/java/org/testng/FileAssert.java b/testng-asserts/src/main/java/org/testng/FileAssert.java
index 5c80dd333e..d6f91af8fe 100644
--- a/testng-asserts/src/main/java/org/testng/FileAssert.java
+++ b/testng-asserts/src/main/java/org/testng/FileAssert.java
@@ -2,6 +2,7 @@
 
 import java.io.File;
 import java.io.IOException;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
  * Assertion tool for File centric assertions. Conceptually, this is an extension of {@link Assert}.
@@ -25,7 +26,7 @@ private FileAssert() {
    * @param tstvalue the file to evaluate
    * @param message the assertion error message
    */
-  public static void assertDirectory(File tstvalue, String message) {
+  public static void assertDirectory(File tstvalue, @Nullable String message) {
     boolean condition = false;
     try {
       condition = tstvalue.isDirectory();
@@ -77,7 +78,7 @@ public static void assertFile(File tstvalue) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertLength(File tstvalue, long expected, String message) {
+  public static void assertLength(File tstvalue, long expected, @Nullable String message) {
     long actual = -1L;
     try {
       actual = tstvalue.isDirectory() ? tstvalue.list().length : tstvalue.length();
@@ -107,7 +108,7 @@ public static void assertLength(File tstvalue, long expected) {
    * @param expected the expected value
    * @param message the assertion error message
    */
-  public static void assertMinLength(File tstvalue, long expected, String message) {
+  public static void assertMinLength(File tstvalue, long expected, @Nullable String message) {
     long actual = -1L;
     try {
       actual = tstvalue.isDirectory() ? tstvalue.list().length : tstvalue.length();
@@ -138,7 +139,7 @@ public static void assertMinLength(File tstvalue, long expected) {
    * @param expected The expected max length
    * @param message the assertion error message
    */
-  public static void assertMaxLength(File tstvalue, long expected, String message) {
+  public static void assertMaxLength(File tstvalue, long expected, @Nullable String message) {
     long actual = -1L;
     try {
       actual = tstvalue.isDirectory() ? tstvalue.list().length : tstvalue.length();
@@ -167,7 +168,7 @@ public static void assertMaxLength(File tstvalue, long expected) {
    * @param tstvalue the file to evaluate
    * @param message the assertion error message
    */
-  public static void assertReadable(File tstvalue, String message) {
+  public static void assertReadable(File tstvalue, @Nullable String message) {
     boolean condition = false;
     try {
       condition = tstvalue.canRead();
@@ -221,7 +222,7 @@ public static void assertWriteable(File tstvalue) {
    * @param tstvalue the file to evaluate
    * @param message the assertion error message
    */
-  public static void assertReadWrite(File tstvalue, String message) {
+  public static void assertReadWrite(File tstvalue, @Nullable String message) {
     boolean condition = false;
     try {
       condition = tstvalue.canRead() && tstvalue.canWrite();
@@ -259,7 +260,7 @@ public static void fail(String message, Throwable realCause) {
    *
    * @param message the assertion error message
    */
-  public static void fail(String message) {
+  public static void fail(@Nullable String message) {
     throw new AssertionError(message);
   }
 
@@ -269,7 +270,8 @@ public static void fail() {
   }
 
   /** Formats failure for file assertions. */
-  private static void failFile(File path, String actual, String expected, String message) {
+  private static void failFile(
+      File path, String actual, @Nullable String expected, @Nullable String message) {
     String formatted = "";
     if (message != null) {
       formatted = message + " ";
@@ -291,7 +293,7 @@ private static void failFile(File path, String actual, String expected, String m
    * @param message
    */
   private static void failSecurity(
-      Exception e, File path, String actual, String expected, String message) {
+      Exception e, File path, String actual, String expected, @Nullable String message) {
     String formatted = "";
     if (message != null) {
       formatted = message + " ";
diff --git a/testng-asserts/src/main/java/org/testng/asserts/Assertion.java b/testng-asserts/src/main/java/org/testng/asserts/Assertion.java
index a3778de3f5..6c2afff713 100644
--- a/testng-asserts/src/main/java/org/testng/asserts/Assertion.java
+++ b/testng-asserts/src/main/java/org/testng/asserts/Assertion.java
@@ -3,6 +3,7 @@
 import java.util.Collection;
 import java.util.Map;
 import java.util.Set;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 /** An assert class with various hooks allowing its behavior to be modified by subclasses. */
 public class Assertion implements IAssertLifecycle {
@@ -41,19 +42,19 @@ public void onBeforeAssert(IAssert<?> assertCommand) {}
   public void onAfterAssert(IAssert<?> assertCommand) {}
 
   private abstract static class SimpleAssert<T> implements IAssert<T> {
-    private final T actual;
-    private final T expected;
-    private final String m_message;
+    private final @Nullable T actual;
+    private final @Nullable T expected;
+    private final @Nullable String m_message;
 
     public SimpleAssert(String message) {
       this(null, null, message);
     }
 
-    public SimpleAssert(T actual, T expected) {
+    public SimpleAssert(@Nullable T actual, @Nullable T expected) {
       this(actual, expected, null);
     }
 
-    public SimpleAssert(T actual, T expected, String message) {
+    public SimpleAssert(@Nullable T actual, @Nullable T expected, String message) {
       this.actual = actual;
       this.expected = expected;
       m_message = message;
diff --git a/testng-asserts/src/main/java/org/testng/asserts/IAssert.java b/testng-asserts/src/main/java/org/testng/asserts/IAssert.java
index f6b3d42731..43ecd53c95 100644
--- a/testng-asserts/src/main/java/org/testng/asserts/IAssert.java
+++ b/testng-asserts/src/main/java/org/testng/asserts/IAssert.java
@@ -1,11 +1,16 @@
 package org.testng.asserts;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 public interface IAssert<T> {
+  @Nullable
   String getMessage();
 
   void doAssert();
 
+  @Nullable
   T getActual();
 
+  @Nullable
   T getExpected();
 }
diff --git a/testng-asserts/src/main/java/org/testng/asserts/SoftAssert.java b/testng-asserts/src/main/java/org/testng/asserts/SoftAssert.java
index 3746eb07e2..3d71367200 100644
--- a/testng-asserts/src/main/java/org/testng/asserts/SoftAssert.java
+++ b/testng-asserts/src/main/java/org/testng/asserts/SoftAssert.java
@@ -1,6 +1,7 @@
 package org.testng.asserts;
 
 import java.util.Map;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.collections.Maps;
 
 /**
@@ -30,7 +31,7 @@ public void assertAll() {
     assertAll(null);
   }
 
-  public void assertAll(String message) {
+  public void assertAll(@Nullable String message) {
     if (!m_errors.isEmpty()) {
       StringBuilder sb = new StringBuilder(null == message ? DEFAULT_SOFT_ASSERT_MESSAGE : message);
       boolean first = true;
diff --git a/testng-asserts/src/main/java/org/testng/package-info.java b/testng-asserts/src/main/java/org/testng/package-info.java
new file mode 100644
index 0000000000..bf86fddb16
--- /dev/null
+++ b/testng-asserts/src/main/java/org/testng/package-info.java
@@ -0,0 +1,6 @@
+@DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.ALL)
+package org.testng;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.framework.qual.DefaultQualifier;
+import org.checkerframework.framework.qual.TypeUseLocation;
diff --git a/testng-asserts/testng-asserts-build.gradle.kts b/testng-asserts/testng-asserts-build.gradle.kts
index 2e958012d5..7d8a3e137e 100644
--- a/testng-asserts/testng-asserts-build.gradle.kts
+++ b/testng-asserts/testng-asserts-build.gradle.kts
@@ -6,6 +6,7 @@ dependencies {
     implementation(projects.testngCollections) {
         because("Lists.newArrayList")
     }
+    implementation("org.checkerframework:checker-qual:3.36.0")
 
     testImplementation("org.testng:testng:7.3.0") {
         because("core depends on assertions and we need testng to test assertions")
diff --git a/testng-collections/src/main/java/org/testng/collections/Maps.java b/testng-collections/src/main/java/org/testng/collections/Maps.java
index 294f2d2b85..5825759b14 100644
--- a/testng-collections/src/main/java/org/testng/collections/Maps.java
+++ b/testng-collections/src/main/java/org/testng/collections/Maps.java
@@ -1,5 +1,6 @@
 package org.testng.collections;
 
+import com.google.errorprone.annotations.InlineMe;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Hashtable;
@@ -13,6 +14,8 @@ public static <K, V> Map<K, V> newHashMap() {
     return new HashMap<>();
   }
 
+  @Deprecated
+  @InlineMe(replacement = "new Hashtable<>()", imports = "java.util.Hashtable")
   public static <K, V> Map<K, V> newHashtable() {
     return new Hashtable<>();
   }
diff --git a/testng-collections/src/main/java/org/testng/collections/MultiMap.java b/testng-collections/src/main/java/org/testng/collections/MultiMap.java
index 8d2df157f6..47202f8ce7 100644
--- a/testng-collections/src/main/java/org/testng/collections/MultiMap.java
+++ b/testng-collections/src/main/java/org/testng/collections/MultiMap.java
@@ -5,6 +5,8 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicBoolean;
+import org.checkerframework.checker.nullness.qual.KeyFor;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 public abstract class MultiMap<K, V, C extends Collection<V>> {
   protected final Map<K, C> m_objects;
@@ -47,18 +49,17 @@ public boolean containsKey(K k) {
   @Override
   public String toString() {
     StringBuilder result = new StringBuilder();
-    Set<K> indices = keySet();
-    for (K i : indices) {
-      result.append("\n    ").append(i).append(" <-- ");
-      for (Object o : m_objects.get(i)) {
-        result.append(o).append(" ");
+    for (Map.Entry<K, C> entry : m_objects.entrySet()) {
+      result.append("\n    ").append(entry.getKey()).append(" <-- ");
+      for (V v : entry.getValue()) {
+        result.append(v).append(" ");
       }
     }
     return result.toString();
   }
 
   public boolean isEmpty() {
-    return m_objects.size() == 0;
+    return m_objects.isEmpty();
   }
 
   public int size() {
@@ -69,11 +70,11 @@ public boolean remove(K key, V value) {
     return get(key).remove(value);
   }
 
-  public C removeAll(K key) {
+  public @Nullable C removeAll(K key) {
     return m_objects.remove(key);
   }
 
-  public Set<Map.Entry<K, C>> entrySet() {
+  public Set<Map.Entry<@KeyFor("this.m_objects") K, C>> entrySet() {
     return m_objects.entrySet();
   }
 
diff --git a/testng-collections/src/main/java/org/testng/package-info.java b/testng-collections/src/main/java/org/testng/package-info.java
new file mode 100644
index 0000000000..bf86fddb16
--- /dev/null
+++ b/testng-collections/src/main/java/org/testng/package-info.java
@@ -0,0 +1,6 @@
+@DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.ALL)
+package org.testng;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.framework.qual.DefaultQualifier;
+import org.checkerframework.framework.qual.TypeUseLocation;
diff --git a/testng-collections/src/main/java/org/testng/util/Strings.java b/testng-collections/src/main/java/org/testng/util/Strings.java
index 452f56f29d..b837ee7604 100644
--- a/testng-collections/src/main/java/org/testng/util/Strings.java
+++ b/testng-collections/src/main/java/org/testng/util/Strings.java
@@ -2,7 +2,6 @@
 
 import java.util.List;
 import java.util.Map;
-import java.util.Optional;
 import java.util.stream.Collectors;
 import org.testng.collections.Maps;
 
@@ -12,7 +11,10 @@ private Strings() {
   }
 
   public static boolean isNullOrEmpty(String string) {
-    return Optional.ofNullable(string).orElse("").trim().isEmpty();
+    if (string == null) {
+      return true;
+    }
+    return string.isEmpty();
   }
 
   public static boolean isNotNullAndNotEmpty(String string) {
@@ -52,7 +54,7 @@ public static String escapeHtml(String text) {
     return result;
   }
 
-  public static String valueOf(Map<?, ?> m) {
+  public static <K, V> String valueOf(Map<K, V> m) {
     return m.values().stream().map(Object::toString).collect(Collectors.joining(" "));
   }
 
diff --git a/testng-collections/testng-collections-build.gradle.kts b/testng-collections/testng-collections-build.gradle.kts
index 0438ec3bf1..9433ef384e 100644
--- a/testng-collections/testng-collections-build.gradle.kts
+++ b/testng-collections/testng-collections-build.gradle.kts
@@ -1,3 +1,8 @@
 plugins {
     id("testng.java-library")
 }
+
+dependencies {
+    implementation("org.checkerframework:checker-qual:3.36.0")
+    implementation("com.google.errorprone:error_prone_annotations:2.20.0")
+}
diff --git a/testng-core-api/src/main/java/org/testng/IAttributes.java b/testng-core-api/src/main/java/org/testng/IAttributes.java
index 98e925cc9c..7165c61339 100644
--- a/testng-core-api/src/main/java/org/testng/IAttributes.java
+++ b/testng-core-api/src/main/java/org/testng/IAttributes.java
@@ -5,8 +5,9 @@
 /** A trait that is used by all interfaces that lets the user add or remove their own attributes. */
 public interface IAttributes {
   /**
+   * Returns the attribute with the given name.
+   *
    * @param name The name of the attribute to return
-   * @return The attribute
    */
   Object getAttribute(String name);
 
@@ -18,7 +19,7 @@ public interface IAttributes {
    */
   void setAttribute(String name, Object value);
 
-  /** @return all the attributes names. */
+  /** Returns all the attributes names. */
   Set<String> getAttributeNames();
 
   /**
diff --git a/testng-core-api/src/main/java/org/testng/IClass.java b/testng-core-api/src/main/java/org/testng/IClass.java
index adf1df9031..66475f43c4 100644
--- a/testng-core-api/src/main/java/org/testng/IClass.java
+++ b/testng-core-api/src/main/java/org/testng/IClass.java
@@ -6,19 +6,19 @@
 /** <code>IClass</code> represents a test class and a collection of its instances. */
 public interface IClass {
 
-  /** @return this test class name. This is the name of the corresponding Java class. */
+  /** Returns this test class name. This is the name of the corresponding Java class. */
   String getName();
 
-  /** @return the &lt;test&gt; tag this class was found in. */
+  /** Returns the &lt;test&gt; tag this class was found in. */
   XmlTest getXmlTest();
 
-  /** @return the *lt;class&gt; tag this class was found in. */
+  /** Returns the *lt;class&gt; tag this class was found in. */
   XmlClass getXmlClass();
 
-  /** @return its test name if this class implements org.testng.ITest, null otherwise. */
+  /** Returns its test name if this class implements org.testng.ITest, null otherwise. */
   String getTestName();
 
-  /** @return the Java class corresponding to this IClass. */
+  /** Returns the Java class corresponding to this IClass. */
   Class<?> getRealClass();
 
   /**
diff --git a/testng-core-api/src/main/java/org/testng/IObjectFactory2.java b/testng-core-api/src/main/java/org/testng/IObjectFactory2.java
index a690d7117f..b829a3b4d2 100644
--- a/testng-core-api/src/main/java/org/testng/IObjectFactory2.java
+++ b/testng-core-api/src/main/java/org/testng/IObjectFactory2.java
@@ -18,6 +18,6 @@ public interface IObjectFactory2 extends ITestObjectFactory {
    */
   @Deprecated
   default Object newInstance(Class<?> cls) {
-    return newInstance(cls, new Object[0]);
+    return newInstance((Class<Object>) cls, new Object[0]);
   }
 }
diff --git a/testng-core-api/src/main/java/org/testng/ISuite.java b/testng-core-api/src/main/java/org/testng/ISuite.java
index 773f95b541..0401597c73 100644
--- a/testng-core-api/src/main/java/org/testng/ISuite.java
+++ b/testng-core-api/src/main/java/org/testng/ISuite.java
@@ -4,6 +4,7 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.internal.annotations.IAnnotationFinder;
 import org.testng.xml.XmlSuite;
 
@@ -14,25 +15,29 @@
  */
 public interface ISuite extends IAttributes {
 
-  /** @return the name of this suite. */
+  /** Returns the name of this suite. */
   String getName();
 
-  /** @return The results for this suite. */
+  /** Returns the results for this suite. */
   Map<String, ISuiteResult> getResults();
 
-  /** @return The object factory used to create all test instances. */
+  /** Returns the object factory used to create all test instances. */
   ITestObjectFactory getObjectFactory();
 
+  /**
+   * Returns null.
+   *
+   * @deprecated This interface stands deprecated as of TestNG 7.5.0
+   */
   @Deprecated
-  /** @deprecated - This interface stands deprecated as of TestNG 7.5.0 */
-  default IObjectFactory2 getObjectFactory2() {
+  default @Nullable IObjectFactory2 getObjectFactory2() {
     return null;
   }
 
-  /** @return The output directory used for the reports. */
+  /** Returns the output directory used for the reports. */
   String getOutputDirectory();
 
-  /** @return true if the tests must be run in parallel. */
+  /** Returns true if the tests must be run in parallel. */
   String getParallel();
 
   String getParentModule();
@@ -40,8 +45,9 @@ default IObjectFactory2 getObjectFactory2() {
   String getGuiceStage();
 
   /**
+   * Returns the value of this parameter, or null if none was specified.
+   *
    * @param parameterName The name of the parameter
-   * @return The value of this parameter, or null if none was specified.
    */
   String getParameter(String parameterName);
 
@@ -52,18 +58,18 @@ default IObjectFactory2 getObjectFactory2() {
    */
   Map<String, Collection<ITestNGMethod>> getMethodsByGroups();
 
-  /** @return a list of all the methods that were invoked in this suite. */
+  /** Returns a list of all the methods that were invoked in this suite. */
   List<IInvokedMethod> getAllInvokedMethods();
 
-  /** @return All the methods that were not included in this test run. */
+  /** Returns all the methods that were not included in this test run. */
   Collection<ITestNGMethod> getExcludedMethods();
 
   /** Triggers the start of running tests included in the suite. */
   void run();
 
   /**
-   * @return The host where this suite was run, or null if it was run locally. The returned string
-   *     has the form: host:port
+   * Returns the host where this suite was run, or null if it was run locally. The returned string
+   * has the form: host:port
    */
   String getHost();
 
@@ -74,10 +80,10 @@ default IObjectFactory2 getObjectFactory2() {
    */
   SuiteRunState getSuiteState();
 
-  /** @return the annotation finder used for the specified type (JDK5 or javadoc) */
+  /** Returns the annotation finder used for the specified type (JDK5 or javadoc) */
   IAnnotationFinder getAnnotationFinder();
 
-  /** @return The representation of the current XML suite file. */
+  /** Returns the representation of the current XML suite file. */
   XmlSuite getXmlSuite();
 
   void addListener(ITestNGListener listener);
@@ -87,8 +93,8 @@ default IObjectFactory2 getObjectFactory2() {
   void setParentInjector(Injector injector);
 
   /**
-   * @return the total number of methods found in this suite. The presence of factories or data
-   *     providers might cause the actual number of test methods run be bigger than this list.
+   * Returns the total number of methods found in this suite. The presence of factories or data
+   * providers might cause the actual number of test methods run be bigger than this list.
    */
   List<ITestNGMethod> getAllMethods();
 }
diff --git a/testng-core-api/src/main/java/org/testng/ITestContext.java b/testng-core-api/src/main/java/org/testng/ITestContext.java
index 5de1055d5b..c12d2d81da 100644
--- a/testng-core-api/src/main/java/org/testng/ITestContext.java
+++ b/testng-core-api/src/main/java/org/testng/ITestContext.java
@@ -2,6 +2,7 @@
 
 import java.util.Collection;
 import java.util.Date;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.xml.XmlTest;
 
 /**
@@ -13,29 +14,30 @@
  */
 public interface ITestContext extends IAttributes {
 
-  /** @return The name of this test. */
+  /** Returns the name of this test. */
   String getName();
 
-  /** @return When this test started running. */
+  /** Returns when this test started running. */
   Date getStartDate();
 
-  /** @return When this test stopped running. */
+  /** Returns when this test stopped running. */
   Date getEndDate();
 
-  /** @return A list of all the tests that run successfully. */
+  /** Returns a list of all the tests that run successfully. */
   IResultMap getPassedTests();
 
-  /** @return A list of all the tests that were skipped */
+  /** Returns a list of all the tests that were skipped */
   IResultMap getSkippedTests();
 
   /**
-   * @return A list of all the tests that failed but are being ignored because annotated with a
-   *     successPercentage.
+   * Returns a list of all the tests that failed but are being ignored because annotated with a
+   * successPercentage.
    */
   IResultMap getFailedButWithinSuccessPercentageTests();
 
   /**
-   * @return A map of all the tests that failed, indexed by their ITestNGMethod.
+   * Returns a map of all the tests that failed, indexed by their ITestNGMethod.
+   *
    * @see org.testng.ITestNGMethod
    */
   IResultMap getFailedTests();
@@ -43,40 +45,40 @@ public interface ITestContext extends IAttributes {
   /** @return All the groups that are included for this test run. */
   String[] getIncludedGroups();
 
-  /** @return All the groups that are excluded for this test run. */
+  /** Returns all the groups that are excluded for this test run. */
   String[] getExcludedGroups();
 
-  /** @return Where the reports will be generated. */
+  /** Returns where the reports will be generated. */
   String getOutputDirectory();
 
-  /** @return The Suite object that was passed to the runner at start-up. */
+  /** Returns the Suite object that was passed to the runner at start-up. */
   ISuite getSuite();
 
-  /** @return All the test methods that were run. */
+  /** Returns all the test methods that were run. */
   ITestNGMethod[] getAllTestMethods();
 
   /**
-   * @return The host where this test was run, or null if it was run locally. The returned string
-   *     has the form: host:port
+   * Returns the host where this test was run, or null if it was run locally. The returned string
+   * has the form: host:port
    */
   String getHost();
 
-  /** @return All the methods that were not included in this test run. */
+  /** Returns all the methods that were not included in this test run. */
   Collection<ITestNGMethod> getExcludedMethods();
 
-  /** @return The information about the successful configuration method invocations. */
+  /** Returns the information about the successful configuration method invocations. */
   IResultMap getPassedConfigurations();
 
-  /** @return The information about the skipped configuration method invocations. */
+  /** Returns the information about the skipped configuration method invocations. */
   IResultMap getSkippedConfigurations();
 
-  /** @return The information about the failed configuration method invocations. */
+  /** Returns the information about the failed configuration method invocations. */
   IResultMap getFailedConfigurations();
 
-  /** @return the current XmlTest. */
+  /** Returns the current XmlTest. */
   XmlTest getCurrentXmlTest();
 
-  default IInjectorFactory getInjectorFactory() {
+  default @Nullable IInjectorFactory getInjectorFactory() {
     return null;
   }
 }
diff --git a/testng-core-api/src/main/java/org/testng/ITestNGMethod.java b/testng-core-api/src/main/java/org/testng/ITestNGMethod.java
index 0a5ec017f2..f890be6690 100644
--- a/testng-core-api/src/main/java/org/testng/ITestNGMethod.java
+++ b/testng-core-api/src/main/java/org/testng/ITestNGMethod.java
@@ -4,6 +4,7 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.Callable;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.annotations.CustomAttribute;
 import org.testng.internal.ConstructorOrMethod;
 import org.testng.internal.IParameterInfo;
@@ -17,8 +18,8 @@
 public interface ITestNGMethod extends Cloneable {
 
   /**
-   * @return The real class on which this method was declared (can be different from
-   *     getMethod().getDeclaringClass() if the test method was defined in a superclass).
+   * Returns the real class on which this method was declared (can be different from
+   * getMethod().getDeclaringClass() if the test method was defined in a superclass).
    */
   Class getRealClass();
 
@@ -48,16 +49,16 @@ public interface ITestNGMethod extends Cloneable {
   long[] getInstanceHashCodes();
 
   /**
-   * @return The groups this method belongs to, possibly added to the groups declared on the class.
+   * Returns the groups this method belongs to, possibly added to the groups declared on the class.
    */
   String[] getGroups();
 
   /**
-   * @return The groups this method depends on, possibly added to the groups declared on the class.
+   * Returns the groups this method depends on, possibly added to the groups declared on the class.
    */
   String[] getGroupsDependedUpon();
 
-  /** @return If a group was not found. */
+  /** Returns if a group was not found. */
   String getMissingGroup();
 
   void setMissingGroup(String group);
@@ -67,24 +68,24 @@ public interface ITestNGMethod extends Cloneable {
   String[] getAfterGroups();
 
   /**
-   * @return The methods this method depends on, possibly added to the methods declared on the
-   *     class.
+   * Returns the methods this method depends on, possibly added to the methods declared on the
+   * class.
    */
   String[] getMethodsDependedUpon();
 
   /**
-   * @return - The set of methods that are dependent on the current method. This information can
-   *     help in deciding what other TestNG methods will be skipped if the current method fails. If
-   *     the current method is a configuration method, then an empty set is returned.
+   * Returns the set of methods that are dependent on the current method. This information can help
+   * in deciding what other TestNG methods will be skipped if the current method fails. If the
+   * current method is a configuration method, then an empty set is returned.
    */
   default Set<ITestNGMethod> downstreamDependencies() {
     throw new UnsupportedOperationException("Pending implementation");
   }
 
   /**
-   * @return - The set of methods upon which the current method has a dependency. This information
-   *     can help in deciding what all TestNG methods need to pass before the current method can be
-   *     executed. If the current method is a configuration method, then an empty set is returned.
+   * Returns the set of methods upon which the current method has a dependency. This information can
+   * help in deciding what all TestNG methods need to pass before the current method can be
+   * executed. If the current method is a configuration method, then an empty set is returned.
    */
   default Set<ITestNGMethod> upstreamDependencies() {
     throw new UnsupportedOperationException("Pending implementation");
@@ -92,31 +93,31 @@ default Set<ITestNGMethod> upstreamDependencies() {
 
   void addMethodDependedUpon(String methodName);
 
-  /** @return true if this method was annotated with @Test */
+  /** Returns true if this method was annotated with @Test */
   boolean isTest();
 
-  /** @return true if this method was annotated with @Configuration and beforeTestMethod = true */
+  /** Returns true if this method was annotated with @Configuration and beforeTestMethod = true */
   boolean isBeforeMethodConfiguration();
 
-  /** @return true if this method was annotated with @Configuration and beforeTestMethod = false */
+  /** Returns true if this method was annotated with @Configuration and beforeTestMethod = false */
   boolean isAfterMethodConfiguration();
 
-  /** @return true if this method was annotated with @Configuration and beforeClassMethod = true */
+  /** Returns true if this method was annotated with @Configuration and beforeClassMethod = true */
   boolean isBeforeClassConfiguration();
 
-  /** @return true if this method was annotated with @Configuration and beforeClassMethod = false */
+  /** Returns true if this method was annotated with @Configuration and beforeClassMethod = false */
   boolean isAfterClassConfiguration();
 
-  /** @return true if this method was annotated with @Configuration and beforeSuite = true */
+  /** Returns true if this method was annotated with @Configuration and beforeSuite = true */
   boolean isBeforeSuiteConfiguration();
 
-  /** @return true if this method was annotated with @Configuration and afterSuite = true */
+  /** Returns true if this method was annotated with @Configuration and afterSuite = true */
   boolean isAfterSuiteConfiguration();
 
-  /** @return <code>true</code> if this method is a @BeforeTest (@Configuration beforeTest=true) */
+  /** Returns <code>true</code> if this method is a @BeforeTest (@Configuration beforeTest=true) */
   boolean isBeforeTestConfiguration();
 
-  /** @return <code>true</code> if this method is an @AfterTest (@Configuration afterTest=true) */
+  /** Returns <code>true</code> if this method is an @AfterTest (@Configuration afterTest=true) */
   boolean isAfterTestConfiguration();
 
   boolean isBeforeGroupsConfiguration();
@@ -131,20 +132,20 @@ default boolean hasAfterGroupsConfiguration() {
     return false;
   }
 
-  /** @return The timeout in milliseconds. */
+  /** Returns the timeout in milliseconds. */
   long getTimeOut();
 
   void setTimeOut(long timeOut);
 
-  /** @return the number of times this method needs to be invoked. */
+  /** Returns the number of times this method needs to be invoked. */
   int getInvocationCount();
 
   void setInvocationCount(int count);
 
-  /** @return the success percentage for this method (between 0 and 100). */
+  /** Returns the success percentage for this method (between 0 and 100). */
   int getSuccessPercentage();
 
-  /** @return The id of the thread this method was run in. */
+  /** Returns the id of the thread this method was run in. */
   String getId();
 
   void setId(String id);
@@ -154,15 +155,16 @@ default boolean hasAfterGroupsConfiguration() {
   void setDate(long date);
 
   /**
+   * Returns true if this ITestNGMethod can be invoked from within IClass.
+   *
    * @param testClass The test class
-   * @return true if this ITestNGMethod can be invoked from within IClass.
    */
   boolean canRunFromClass(IClass testClass);
 
-  /** @return true if this method is alwaysRun=true */
+  /** Returns true if this method is alwaysRun=true */
   boolean isAlwaysRun();
 
-  /** @return the number of threads to be used when invoking the method on parallel */
+  /** Returns the number of threads to be used when invoking the method on parallel */
   int getThreadPoolSize();
 
   void setThreadPoolSize(int threadPoolSize);
@@ -197,7 +199,7 @@ default boolean hasAfterGroupsConfiguration() {
 
   void setSkipFailedInvocations(boolean skip);
 
-  /** @return The time under which all invocationCount methods need to complete by. */
+  /** Returns the time under which all invocationCount methods need to complete by. */
   long getInvocationTimeOut();
 
   boolean ignoreMissingDependencies();
@@ -239,21 +241,20 @@ default boolean hasAfterGroupsConfiguration() {
 
   void setInterceptedPriority(int priority);
 
-  /** @return the XmlTest this method belongs to. */
+  /** Returns the XmlTest this method belongs to. */
   XmlTest getXmlTest();
 
   ConstructorOrMethod getConstructorOrMethod();
 
   /**
+   * Returns the parameters found in the include tag, if any
+   *
    * @param test - The {@link XmlTest} object.
-   * @return the parameters found in the include tag, if any
    */
   Map<String, String> findMethodParameters(XmlTest test);
 
   /**
-   * getRealClass().getName() + "." + getMethodName()
-   *
-   * @return qualified name for this method
+   * Returns the qualified name for this method: getRealClass().getName() + "." + getMethodName()
    */
   String getQualifiedName();
 
@@ -262,26 +263,26 @@ default boolean isDataDriven() {
   }
 
   /**
-   * @return - A {@link IParameterInfo} object that represents details about the parameters
-   *     associated with the factory method.
+   * Returns a {@link IParameterInfo} object that represents details about the parameters associated
+   * with the factory method.
    */
-  default IParameterInfo getFactoryMethodParamsInfo() {
+  default @Nullable IParameterInfo getFactoryMethodParamsInfo() {
     return null;
   }
 
   /**
-   * @return - An array of {@link CustomAttribute} that represents the custom attributes associated
-   *     with a test.
+   * Returns an array of {@link CustomAttribute} that represents the custom attributes associated
+   * with a test.
    */
   default CustomAttribute[] getAttributes() {
     return new CustomAttribute[] {};
   }
 
   /**
-   * @return - An {@link IDataProviderMethod} for a data provider powered test method and <code>null
+   * Returns an {@link IDataProviderMethod} for a data provider powered test method and <code>null
    *     </code> otherwise.
    */
-  default IDataProviderMethod getDataProviderMethod() {
+  default @Nullable IDataProviderMethod getDataProviderMethod() {
     return null;
   }
 
@@ -290,8 +291,8 @@ default Class<?>[] getParameterTypes() {
   }
 
   /**
-   * @return - <code>true</code> if the configuration failure arising out of this method should be
-   *     ignored.
+   * Returns <code>true</code> if the configuration failure arising out of this method should be
+   * ignored.
    */
   default boolean isIgnoreFailure() {
     return false;
diff --git a/testng-core-api/src/main/java/org/testng/ITestResult.java b/testng-core-api/src/main/java/org/testng/ITestResult.java
index 00649a4615..1d8812fd62 100644
--- a/testng-core-api/src/main/java/org/testng/ITestResult.java
+++ b/testng-core-api/src/main/java/org/testng/ITestResult.java
@@ -3,6 +3,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.internal.thread.ThreadTimeoutException;
 
 /**
@@ -22,76 +23,76 @@ public interface ITestResult extends IAttributes, Comparable<ITestResult> {
   int SUCCESS_PERCENTAGE_FAILURE = 4;
   int STARTED = 16;
 
-  /** @return The status of this result, using one of the constants above. */
+  /** Returns the status of this result, using one of the constants above. */
   int getStatus();
 
   void setStatus(int status);
 
-  /** @return The test method this result represents. */
+  /** Returns the test method this result represents. */
   ITestNGMethod getMethod();
 
-  /** @return The parameters this method was invoked with. */
+  /** Returns the parameters this method was invoked with. */
   Object[] getParameters();
 
   void setParameters(Object[] parameters);
 
-  /** @return The test class used this object is a result for. */
+  /** Returns the test class used this object is a result for. */
   IClass getTestClass();
 
   /**
-   * @return The throwable that was thrown while running the method, or null if no exception was
-   *     thrown.
+   * Returns the throwable that was thrown while running the method, or null if no exception was
+   * thrown.
    */
   Throwable getThrowable();
 
   void setThrowable(Throwable throwable);
 
-  /** @return the start date for this test, in milliseconds. */
+  /** Returns the start date for this test, in milliseconds. */
   long getStartMillis();
 
-  /** @return the end date for this test, in milliseconds. */
+  /** Returns the end date for this test, in milliseconds. */
   long getEndMillis();
 
   void setEndMillis(long millis);
 
-  /** @return The name of this TestResult, typically identical to the name of the method. */
+  /** Returns the name of this TestResult, typically identical to the name of the method. */
   String getName();
 
-  /** @return true if if this test run is a SUCCESS */
+  /** Returns true if if this test run is a SUCCESS */
   boolean isSuccess();
 
   /**
-   * @return The host where this suite was run, or null if it was run locally. The returned string
-   *     has the form: host:port
+   * Returns the host where this suite was run, or null if it was run locally. The returned string
+   * has the form: host:port
    */
   String getHost();
 
-  /** @return The instance on which this method was run. */
+  /** Returns the instance on which this method was run. */
   Object getInstance();
 
   /**
-   * @return - A parameter array that was passed to a factory method (or) an empty object array
-   *     otherwise.
+   * Returns a parameter array that was passed to a factory method (or) an empty object array
+   * otherwise.
    */
   Object[] getFactoryParameters();
 
   /**
-   * @return The test name if this result's related instance implements ITest or
-   *     use @Test(testName=...), null otherwise.
+   * Returns the test name if this result's related instance implements ITest or
+   * use @Test(testName=...), null otherwise.
    */
   String getTestName();
 
   String getInstanceName();
 
-  /** @return the {@link ITestContext} for this test result. */
+  /** Returns the {@link ITestContext} for this test result. */
   ITestContext getTestContext();
 
   /** @param name - The new name to be used as a test name */
   void setTestName(String name);
 
   /**
-   * @return - <code>true</code> if the test was retried again by an implementation of {@link
-   *     IRetryAnalyzer}
+   * Returns <code>true</code> if the test was retried again by an implementation of {@link
+   * IRetryAnalyzer}.
    */
   boolean wasRetried();
 
@@ -101,47 +102,49 @@ public interface ITestResult extends IAttributes, Comparable<ITestResult> {
   void setWasRetried(boolean wasRetried);
 
   /**
-   * @return - The list of either upstream method(s) or configuration method(s) whose failure led to
-   *     the current method being skipped. An empty list is returned when the current method is not
-   *     a skipped method.
+   * Returns the list of either upstream method(s) or configuration method(s) whose failure led to
+   * the current method being skipped. An empty list is returned when the current method is not a
+   * skipped method.
    */
   default List<ITestNGMethod> getSkipCausedBy() {
     return Collections.emptyList();
   }
 
   /**
-   * @return - A unique id for the current JVM that represents a unique way of identifying a
-   *     specific test method's result.
+   * Returns a unique id for the current JVM that represents a unique way of identifying a specific
+   * test method's result.
    */
   String id();
 
   /**
-   * @return - <code>true</code> if the current test result is either {@link ITestResult#STARTED} or
-   *     {@link ITestResult#CREATED}
+   * Returns <code>true</code> if the current test result is either {@link ITestResult#STARTED} or
+   * {@link ITestResult#CREATED}
    */
   default boolean isNotRunning() {
     return getStatus() == STARTED || getStatus() == CREATED;
   }
 
   /**
-   * @return - A list of all user facing statuses viz.,
-   *     <ul>
-   *       <li>{@link ITestResult#SUCCESS}
-   *       <li>{@link ITestResult#SUCCESS_PERCENTAGE_FAILURE}
-   *       <li>{@link ITestResult#FAILURE}
-   *       <li>{@link ITestResult#SKIP}
-   *     </ul>
+   * Returns a list of all user facing statuses viz.,
+   *
+   * <ul>
+   *   <li>{@link ITestResult#SUCCESS}
+   *   <li>{@link ITestResult#SUCCESS_PERCENTAGE_FAILURE}
+   *   <li>{@link ITestResult#FAILURE}
+   *   <li>{@link ITestResult#SKIP}
+   * </ul>
    */
   static List<String> finalStatuses() {
     return Arrays.asList("SUCCESS", "FAILURE", "SKIP", "SUCCESS_PERCENTAGE_FAILURE");
   }
 
   /**
+   * Returns <code>true</code> if the test failure was due to a timeout.
+   *
    * @param result - The test result of a method
-   * @return - <code>true</code> if the test failure was due to a timeout.
    */
   static boolean wasFailureDueToTimeout(ITestResult result) {
-    Throwable cause = result.getThrowable();
+    @Nullable Throwable cause = result.getThrowable();
     while (cause != null && !cause.getClass().equals(Throwable.class)) {
       if (cause instanceof ThreadTimeoutException) {
         return true;
diff --git a/testng-core-api/src/main/java/org/testng/Reporter.java b/testng-core-api/src/main/java/org/testng/Reporter.java
index e27356b6db..6d84975f62 100644
--- a/testng-core-api/src/main/java/org/testng/Reporter.java
+++ b/testng-core-api/src/main/java/org/testng/Reporter.java
@@ -3,6 +3,7 @@
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.collections.Lists;
 import org.testng.collections.Maps;
 import org.testng.internal.Utils;
@@ -27,7 +28,7 @@ public class Reporter {
   // When tests are run in parallel, each thread may be working with different
   // 'current test result'. Also, this value should be inherited if the test code
   // spawns its own thread.
-  private static final ThreadLocal<ITestResult> m_currentTestResult =
+  private static final ThreadLocal<@Nullable ITestResult> m_currentTestResult =
       new InheritableThreadLocal<>();
 
   /** All output logged in a sequential order. */
@@ -153,7 +154,7 @@ public static void log(String s, int level) {
   }
 
   /** @return the current test result. */
-  public static ITestResult getCurrentTestResult() {
+  public static @Nullable ITestResult getCurrentTestResult() {
     return m_currentTestResult.get();
   }
 
diff --git a/testng-core-api/src/main/java/org/testng/TestNGException.java b/testng-core-api/src/main/java/org/testng/TestNGException.java
index fd1e31c9bd..38f2d15356 100644
--- a/testng-core-api/src/main/java/org/testng/TestNGException.java
+++ b/testng-core-api/src/main/java/org/testng/TestNGException.java
@@ -1,5 +1,7 @@
 package org.testng;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 /** The base class for all exceptions thrown by TestNG. */
 public class TestNGException extends RuntimeException {
 
@@ -13,7 +15,7 @@ public TestNGException(String string) {
     super("\n" + string);
   }
 
-  public TestNGException(String string, Throwable t) {
+  public TestNGException(String string, @Nullable Throwable t) {
     super("\n" + string, t);
   }
 }
diff --git a/testng-core-api/src/main/java/org/testng/internal/ClassHelper.java b/testng-core-api/src/main/java/org/testng/internal/ClassHelper.java
index 691992e62a..110c05e527 100644
--- a/testng-core-api/src/main/java/org/testng/internal/ClassHelper.java
+++ b/testng-core-api/src/main/java/org/testng/internal/ClassHelper.java
@@ -12,6 +12,7 @@
 import java.util.Vector;
 import java.util.function.BiConsumer;
 import java.util.stream.Collectors;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.TestNGException;
 import org.testng.annotations.IFactoryAnnotation;
 import org.testng.collections.Lists;
@@ -52,7 +53,7 @@ public static void addClassLoader(final ClassLoader loader) {
 
   static List<ClassLoader> appendContextualClassLoaders(List<ClassLoader> currentLoaders) {
     List<ClassLoader> allClassLoaders = Lists.newArrayList();
-    ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
+    @Nullable ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
     if (contextClassLoader != null) {
       allClassLoaders.add(contextClassLoader);
     }
@@ -68,7 +69,7 @@ static List<ClassLoader> appendContextualClassLoaders(List<ClassLoader> currentL
    * @param className the class name to be loaded.
    * @return the class or null if the class is not found.
    */
-  public static Class<?> forName(final String className) {
+  public static @Nullable Class<?> forName(final String className) {
     List<ClassLoader> allClassLoaders = appendContextualClassLoaders(classLoaders);
 
     for (ClassLoader classLoader : allClassLoaders) {
@@ -171,21 +172,19 @@ public static Set<Method> getAvailableMethods(Class<?> clazz) {
       appendMethod(methods, declaredMethod);
     }
 
-    Class<?> parent = clazz.getSuperclass();
-    if (null != parent) {
-      while (!Object.class.equals(parent)) {
-        Set<Map.Entry<String, Set<Method>>> extractedMethods =
-            extractMethods(clazz, parent, methods).entrySet();
-        for (Map.Entry<String, Set<Method>> extractedMethod : extractedMethods) {
-          Set<Method> m = methods.get(extractedMethod.getKey());
-          if (m == null) {
-            methods.put(extractedMethod.getKey(), extractedMethod.getValue());
-          } else {
-            m.addAll(extractedMethod.getValue());
-          }
+    @Nullable Class<?> parent = clazz.getSuperclass();
+    while (parent != null && !Object.class.equals(parent)) {
+      Set<Map.Entry<String, Set<Method>>> extractedMethods =
+          extractMethods(clazz, parent, methods).entrySet();
+      for (Map.Entry<String, Set<Method>> extractedMethod : extractedMethods) {
+        Set<Method> m = methods.get(extractedMethod.getKey());
+        if (m == null) {
+          methods.put(extractedMethod.getKey(), extractedMethod.getValue());
+        } else {
+          m.addAll(extractedMethod.getValue());
         }
-        parent = parent.getSuperclass();
       }
+      parent = parent.getSuperclass();
     }
 
     Set<Method> returnValue = Sets.newHashSet();
@@ -231,7 +230,8 @@ private static boolean canInclude(
     return visible && hasNoInheritanceTraits;
   }
 
-  private static boolean isSamePackage(Package childPackage, Package classPackage) {
+  private static boolean isSamePackage(
+      @Nullable Package childPackage, @Nullable Package classPackage) {
     boolean isSamePackage = false;
 
     if ((null == childPackage) && (null == classPackage)) {
diff --git a/testng-core-api/src/main/java/org/testng/internal/ConstructorOrMethod.java b/testng-core-api/src/main/java/org/testng/internal/ConstructorOrMethod.java
index fcdd85b3db..1743d9b4dd 100644
--- a/testng-core-api/src/main/java/org/testng/internal/ConstructorOrMethod.java
+++ b/testng-core-api/src/main/java/org/testng/internal/ConstructorOrMethod.java
@@ -4,20 +4,23 @@
 import java.lang.reflect.Executable;
 import java.lang.reflect.Method;
 import java.util.Objects;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 /** Encapsulation of either a method or a constructor. */
 public class ConstructorOrMethod {
 
-  private Method m_method;
-  private Constructor<?> m_constructor;
+  private @Nullable Method m_method;
+  private @Nullable Constructor<?> m_constructor;
   private boolean m_enabled = true;
 
   public ConstructorOrMethod(Method m) {
     m_method = m;
+    m_constructor = null;
   }
 
   public ConstructorOrMethod(Constructor<?> c) {
     m_constructor = c;
+    m_method = null;
   }
 
   public ConstructorOrMethod(Executable e) {
@@ -44,11 +47,11 @@ public Class<?>[] getParameterTypes() {
         : getConstructor().getParameterTypes();
   }
 
-  public Method getMethod() {
+  public @Nullable Method getMethod() {
     return m_method;
   }
 
-  public Constructor<?> getConstructor() {
+  public @Nullable Constructor<?> getConstructor() {
     return m_constructor;
   }
 
@@ -60,7 +63,7 @@ private Executable getInternalConstructorOrMethod() {
   }
 
   @Override
-  public boolean equals(Object o) {
+  public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/testng-core-api/src/main/java/org/testng/internal/PackageUtils.java b/testng-core-api/src/main/java/org/testng/internal/PackageUtils.java
index 6669fb6ab4..347d4001f2 100644
--- a/testng-core-api/src/main/java/org/testng/internal/PackageUtils.java
+++ b/testng-core-api/src/main/java/org/testng/internal/PackageUtils.java
@@ -16,6 +16,7 @@
 import java.util.function.Function;
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.collections.Lists;
 import org.testng.internal.protocols.Input;
 import org.testng.internal.protocols.Processor;
@@ -29,7 +30,7 @@
  * @author <a href="mailto:cedric@beust.com">Cedric Beust</a>
  */
 public class PackageUtils {
-  private static String[] testClassPaths;
+  private static @Nullable String[] testClassPaths;
 
   /** The additional class loaders to find classes in. */
   private static final Collection<ClassLoader> classLoaders = new ConcurrentLinkedDeque<>();
@@ -79,12 +80,12 @@ public static String[] findClassesInPackage(
         .toArray(String[]::new);
   }
 
-  private static String[] getTestClasspath() {
+  private static @Nullable String[] getTestClasspath() {
     if (null != testClassPaths) {
       return testClassPaths;
     }
 
-    String testClasspath = RuntimeBehavior.getTestClasspath();
+    @Nullable String testClasspath = RuntimeBehavior.getTestClasspath();
     if (null == testClasspath) {
       return null;
     }
@@ -124,7 +125,7 @@ private static Function<ClassLoader, Stream<URL>> asURLs(String packageDir) {
   }
 
   private static boolean matchTestClasspath(URL url, String lastFragment, boolean recursive) {
-    String[] classpathFragments = getTestClasspath();
+    @Nullable String[] classpathFragments = getTestClasspath();
     if (null == classpathFragments) {
       return true;
     }
diff --git a/testng-core-api/src/main/java/org/testng/internal/PropertyUtils.java b/testng-core-api/src/main/java/org/testng/internal/PropertyUtils.java
index 29b53b18be..3e483a74de 100644
--- a/testng-core-api/src/main/java/org/testng/internal/PropertyUtils.java
+++ b/testng-core-api/src/main/java/org/testng/internal/PropertyUtils.java
@@ -8,6 +8,7 @@
 import java.beans.PropertyDescriptor;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.TestNGException;
 import org.testng.log4testng.Logger;
 
@@ -21,7 +22,7 @@ public class PropertyUtils {
   private static final Logger LOGGER = Logger.getLogger(PropertyUtils.class);
 
   @SuppressWarnings("unchecked")
-  public static <T> T convertType(Class<T> type, String value, String paramName) {
+  public static <T> @Nullable T convertType(Class<T> type, String value, String paramName) {
     try {
       if (value == null || NULL_VALUE.equalsIgnoreCase(value)) {
         if (type.isPrimitive()) {
@@ -70,14 +71,14 @@ public static <T> T convertType(Class<T> type, String value, String paramName) {
     throw new TestNGException("Unsupported type parameter : " + type);
   }
 
-  public static void setProperty(Object instance, String name, String value) {
+  public static void setProperty(@Nullable Object instance, String name, String value) {
     if (instance == null) {
       LOGGER.warn(
           "Cannot set property " + name + " with value " + value + ". The target instance is null");
       return;
     }
 
-    Class<?> propClass = getPropertyType(instance.getClass(), name);
+    @Nullable Class<?> propClass = getPropertyType(instance.getClass(), name);
     if (propClass == null) {
       LOGGER.warn(
           "Cannot set property "
@@ -88,64 +89,71 @@ public static void setProperty(Object instance, String name, String value) {
       return;
     }
 
-    Object realValue = convertType(propClass, value, name);
+    @Nullable Object realValue = convertType(propClass, value, name);
     // TODO: Here the property desc is searched again
     setPropertyRealValue(instance, name, realValue);
   }
 
-  public static Class<?> getPropertyType(Class<?> instanceClass, String propertyName) {
+  public static @Nullable Class<?> getPropertyType(
+      @Nullable Class<?> instanceClass, String propertyName) {
     if (instanceClass == null) {
       LOGGER.warn(
           "Cannot retrieve property class for " + propertyName + ". Target instance class is null");
     }
-    PropertyDescriptor propDesc = getPropertyDescriptor(instanceClass, propertyName);
+    @Nullable PropertyDescriptor propDesc = getPropertyDescriptor(instanceClass, propertyName);
     if (propDesc == null) {
       return null;
     }
     return propDesc.getPropertyType();
   }
 
-  private static PropertyDescriptor getPropertyDescriptor(
-      Class<?> targetClass, String propertyName) {
-    PropertyDescriptor result = null;
+  private static @Nullable PropertyDescriptor getPropertyDescriptor(
+      @Nullable Class<?> targetClass, String propertyName) {
     if (targetClass == null) {
       LOGGER.warn("Cannot retrieve property " + propertyName + ". Class is null");
-    } else {
-      try {
-        BeanInfo beanInfo = Introspector.getBeanInfo(targetClass);
-        PropertyDescriptor[] propDescriptors = beanInfo.getPropertyDescriptors();
-        for (PropertyDescriptor propDesc : propDescriptors) {
-          if (propDesc.getName().equals(propertyName)) {
-            result = propDesc;
-            break;
-          }
+      return null;
+    }
+    @Nullable PropertyDescriptor result = null;
+    try {
+      BeanInfo beanInfo = Introspector.getBeanInfo(targetClass);
+      PropertyDescriptor[] propDescriptors = beanInfo.getPropertyDescriptors();
+      for (PropertyDescriptor propDesc : propDescriptors) {
+        if (propDesc.getName().equals(propertyName)) {
+          result = propDesc;
+          break;
         }
-      } catch (IntrospectionException ie) {
-        LOGGER.warn("Cannot retrieve property " + propertyName + ". Cause is: " + ie);
       }
+    } catch (IntrospectionException ie) {
+      LOGGER.warn("Cannot retrieve property " + propertyName + ". Cause is: " + ie);
     }
     return result;
   }
 
-  public static void setPropertyRealValue(Object instance, String name, Object value) {
+  public static void setPropertyRealValue(
+      @Nullable Object instance, String name, @Nullable Object value) {
     if (instance == null) {
       LOGGER.warn(
           "Cannot set property " + name + " with value " + value + ". Target instance is null");
       return;
     }
 
-    PropertyDescriptor propDesc = getPropertyDescriptor(instance.getClass(), name);
+    @Nullable PropertyDescriptor propDesc = getPropertyDescriptor(instance.getClass(), name);
     if (propDesc == null) {
       LOGGER.warn(
           "Cannot set property " + name + " with value " + value + ". Property does not exist");
       return;
     }
 
-    Method method = propDesc.getWriteMethod();
-    try {
-      method.invoke(instance, value);
-    } catch (IllegalAccessException | InvocationTargetException iae) {
-      LOGGER.warn("Cannot set property " + name + " with value " + value + ". Cause " + iae);
+    @Nullable Method method = propDesc.getWriteMethod();
+    if (method == null) {
+      LOGGER.warn(
+          "Cannot set property " + name + " with value " + value + ". Cause method not writeable");
+    } else {
+      try {
+        method.invoke(instance, value);
+      } catch (IllegalAccessException | InvocationTargetException iae) {
+        LOGGER.warn("Cannot set property " + name + " with value " + value + ". Cause " + iae);
+      }
     }
   }
 }
diff --git a/testng-core-api/src/main/java/org/testng/internal/ReporterConfig.java b/testng-core-api/src/main/java/org/testng/internal/ReporterConfig.java
index 255550fb74..c51b16d205 100644
--- a/testng-core-api/src/main/java/org/testng/internal/ReporterConfig.java
+++ b/testng-core-api/src/main/java/org/testng/internal/ReporterConfig.java
@@ -1,6 +1,7 @@
 package org.testng.internal;
 
 import java.util.List;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.collections.Lists;
 
 /** Stores the information regarding the configuration of a pluggable report listener. */
@@ -43,7 +44,7 @@ public String serialize() {
     return sb.toString();
   }
 
-  public static ReporterConfig deserialize(String inputString) {
+  public static @Nullable ReporterConfig deserialize(String inputString) {
 
     if (Utils.isStringEmpty(inputString)) {
       return null;
diff --git a/testng-core-api/src/main/java/org/testng/internal/RuntimeBehavior.java b/testng-core-api/src/main/java/org/testng/internal/RuntimeBehavior.java
index 4879e57623..840466f5dd 100644
--- a/testng-core-api/src/main/java/org/testng/internal/RuntimeBehavior.java
+++ b/testng-core-api/src/main/java/org/testng/internal/RuntimeBehavior.java
@@ -4,6 +4,7 @@
 import java.util.List;
 import java.util.Optional;
 import java.util.TimeZone;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 /** This class houses handling all JVM arguments by TestNG */
 public final class RuntimeBehavior {
@@ -94,7 +95,7 @@ public static String orderMethodsBasedOn() {
     return System.getProperty("testng.order", Systematiser.Order.INSTANCES.getValue());
   }
 
-  public static String getTestClasspath() {
+  public static @Nullable String getTestClasspath() {
     return System.getProperty(TEST_CLASSPATH);
   }
 
diff --git a/testng-core-api/src/main/java/org/testng/internal/Systematiser.java b/testng-core-api/src/main/java/org/testng/internal/Systematiser.java
index 183a562821..cff5a85ca6 100644
--- a/testng-core-api/src/main/java/org/testng/internal/Systematiser.java
+++ b/testng-core-api/src/main/java/org/testng/internal/Systematiser.java
@@ -3,6 +3,7 @@
 import java.util.Arrays;
 import java.util.Comparator;
 import java.util.Objects;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.ITestNGMethod;
 
 /** Helps determine how should {@link ITestNGMethod} be ordered by TestNG. */
@@ -15,7 +16,7 @@ public final class Systematiser {
           .thenComparing(Object::toString)
           .thenComparing(
               method -> {
-                IParameterInfo paramsInfo = method.getFactoryMethodParamsInfo();
+                @Nullable IParameterInfo paramsInfo = method.getFactoryMethodParamsInfo();
                 // TODO: avoid toString in parameter comparison
                 return paramsInfo == null ? "" : Arrays.toString(paramsInfo.getParameters());
               })
diff --git a/testng-core-api/src/main/java/org/testng/internal/Utils.java b/testng-core-api/src/main/java/org/testng/internal/Utils.java
index 1ee0850721..d5e7e1ac2f 100644
--- a/testng-core-api/src/main/java/org/testng/internal/Utils.java
+++ b/testng-core-api/src/main/java/org/testng/internal/Utils.java
@@ -18,7 +18,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.stream.Collectors;
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.ITestNGMethod;
 import org.testng.TestNGException;
 import org.testng.collections.Lists;
@@ -131,7 +131,7 @@ public static void writeFile(@Nullable String outputDir, String fileName, String
    */
   private static void writeFile(
       @Nullable File outputFolder, String fileNameParameter, String sb, @Nullable String encoding) {
-    File outDir = outputFolder;
+    @Nullable File outDir = outputFolder;
     String fileName = fileNameParameter;
     try {
       if (outDir == null) {
@@ -224,7 +224,7 @@ public static void log(String msg) {
    * @param level the logging level of the message.
    * @param msg the message to log to System.out.
    */
-  public static void log(String cls, int level, String msg) {
+  public static void log(String cls, int level, @Nullable String msg) {
     // Why this coupling on a static member of getVerbose()?
     if (getVerbose() >= level) {
       if (cls.length() > 0) {
@@ -244,8 +244,8 @@ public static void warn(String warnMsg) {
   }
 
   /* Tokenize the string using the separator. */
-  public static String[] split(String string, String sep) {
-    if ((string == null) || (string.length() == 0)) {
+  public static String[] split(@Nullable String string, String sep) {
+    if (string == null || string.isEmpty()) {
       return new String[0];
     }
 
@@ -270,12 +270,12 @@ public static String[] split(String string, String sep) {
 
   public static void writeResourceToFile(File file, String resourceName, Class<?> clasz)
       throws IOException {
-    InputStream inputStream = clasz.getResourceAsStream("/" + resourceName);
+    @Nullable InputStream inputStream = clasz.getResourceAsStream("/" + resourceName);
     if (inputStream == null) {
       LOG.error("Couldn't find resource on the class path: " + resourceName);
       return;
     }
-    try {
+    try (inputStream) {
       try (FileOutputStream outputStream = new FileOutputStream(file)) {
         int nread;
         byte[] buffer = new byte[4096];
@@ -283,28 +283,26 @@ public static void writeResourceToFile(File file, String resourceName, Class<?>
           outputStream.write(buffer, 0, nread);
         }
       }
-    } finally {
-      inputStream.close();
     }
   }
 
-  public static String defaultIfStringEmpty(String s, String defaultValue) {
+  public static String defaultIfStringEmpty(@Nullable String s, String defaultValue) {
     return isStringEmpty(s) ? defaultValue : s;
   }
 
-  public static boolean isStringBlank(String s) {
-    return s == null || "".equals(s.trim());
+  public static boolean isStringBlank(@Nullable String s) {
+    return s == null || s.trim().isEmpty();
   }
 
-  public static boolean isStringEmpty(String s) {
-    return s == null || "".equals(s);
+  public static boolean isStringEmpty(@Nullable String s) {
+    return s == null || s.isEmpty();
   }
 
-  public static boolean isStringNotBlank(String s) {
+  public static boolean isStringNotBlank(@Nullable String s) {
     return !isStringBlank(s);
   }
 
-  public static boolean isStringNotEmpty(String s) {
+  public static boolean isStringNotEmpty(@Nullable String s) {
     return !isStringEmpty(s);
   }
 
@@ -360,7 +358,7 @@ private enum StackTraceType {
     FULL
   }
 
-  public static String escapeHtml(String s) {
+  public static @Nullable String escapeHtml(@Nullable String s) {
     if (s == null) {
       return null;
     }
@@ -369,7 +367,7 @@ public static String escapeHtml(String s) {
 
     for (int i = 0; i < s.length(); i++) {
       char c = s.charAt(i);
-      String nc = ESCAPES.get(c);
+      @Nullable String nc = ESCAPES.get(c);
       if (nc != null) {
         result.append(nc);
       } else {
@@ -381,10 +379,6 @@ public static String escapeHtml(String s) {
   }
 
   public static String escapeUnicode(String s) {
-    if (s == null) {
-      return null;
-    }
-
     StringBuilder result = new StringBuilder();
 
     for (int i = 0; i < s.length(); i++) {
@@ -403,11 +397,11 @@ static String filterTrace(String trace) {
 
     try {
       // first line contains the thrown exception
-      String line = bufferedReader.readLine();
-      if (line == null) {
+      @Nullable String firstLine = bufferedReader.readLine();
+      if (firstLine == null) {
         return "";
       }
-      buf.append(line).append(LINE_SEP);
+      buf.append(firstLine).append(LINE_SEP);
 
       //
       // the stack frames of the trace
@@ -415,6 +409,7 @@ static String filterTrace(String trace) {
       String[] excludedStrings =
           new String[] {"org.testng", "reflect", "org.gradle", "org.apache.maven.surefire"};
 
+      String line;
       int excludedCount = 0;
       while ((line = bufferedReader.readLine()) != null) {
         boolean isExcluded = false;
diff --git a/testng-core-api/src/main/java/org/testng/internal/objects/InstanceCreator.java b/testng-core-api/src/main/java/org/testng/internal/objects/InstanceCreator.java
index a293981af8..6e5e244429 100644
--- a/testng-core-api/src/main/java/org/testng/internal/objects/InstanceCreator.java
+++ b/testng-core-api/src/main/java/org/testng/internal/objects/InstanceCreator.java
@@ -2,6 +2,7 @@
 
 import java.lang.reflect.Constructor;
 import java.lang.reflect.InvocationTargetException;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.TestNGException;
 import org.testng.internal.ClassHelper;
 
@@ -15,7 +16,10 @@ private InstanceCreator() {
   }
 
   public static <T> T newInstance(String className, Object... parameters) {
-    Class<?> clazz = ClassHelper.forName(className);
+    @Nullable Class<?> clazz = ClassHelper.forName(className);
+    if (clazz == null) {
+      throw new TestNGException(className + " was not found.");
+    }
     return (T) newInstance(clazz, parameters);
   }
 
@@ -43,7 +47,7 @@ public static <T> T newInstance(Constructor<T> constructor, Object... parameters
   }
 
   public static <T> T newInstance(Class<T> cls, Object... parameters) {
-    Constructor<T> ctor = null;
+    @Nullable Constructor<T> ctor = null;
     for (Constructor<?> c : cls.getConstructors()) {
       // Just comparing parameter array sizes. Comparing the parameter types
       // is more error prone since we need to take conversions into account
diff --git a/testng-core-api/src/main/java/org/testng/internal/protocols/BundledResourceProcessor.java b/testng-core-api/src/main/java/org/testng/internal/protocols/BundledResourceProcessor.java
index 79d0626b8c..bb55d65cfb 100644
--- a/testng-core-api/src/main/java/org/testng/internal/protocols/BundledResourceProcessor.java
+++ b/testng-core-api/src/main/java/org/testng/internal/protocols/BundledResourceProcessor.java
@@ -7,6 +7,7 @@
 import java.net.URLConnection;
 import java.net.URLDecoder;
 import java.util.List;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.collections.Lists;
 
 class BundledResourceProcessor extends Processor {
@@ -32,7 +33,10 @@ private static List<String> processBundledResources(
       URLConnection connection = url.openConnection();
       Method thisMethod = url.openConnection().getClass().getDeclaredMethod("getFileURL", params);
       Object[] paramsObj = {};
-      URL fileUrl = (URL) thisMethod.invoke(connection, paramsObj);
+      @Nullable URL fileUrl = (URL) thisMethod.invoke(connection, paramsObj);
+      if (fileUrl == null) {
+        return Lists.newArrayList();
+      }
       return findClassesInDirPackage(
           packageOnly, included, excluded, URLDecoder.decode(fileUrl.getFile(), UTF_8), recursive);
     } catch (Exception ex) {
diff --git a/testng-core-api/src/main/java/org/testng/internal/protocols/Input.java b/testng-core-api/src/main/java/org/testng/internal/protocols/Input.java
index 18fd4c23b0..1eb8904e23 100644
--- a/testng-core-api/src/main/java/org/testng/internal/protocols/Input.java
+++ b/testng-core-api/src/main/java/org/testng/internal/protocols/Input.java
@@ -53,7 +53,14 @@ public static final class Builder {
     private String packageDirName;
     private String packageName;
 
-    private Builder() {}
+    private Builder() {
+      included = Collections.emptyList();
+      excluded = Collections.emptyList();
+      packageWithoutWildCards = "";
+      recursive = false;
+      packageDirName = "";
+      packageName = "";
+    }
 
     public static Builder newBuilder() {
       return new Builder();
diff --git a/testng-core-api/src/main/java/org/testng/log4testng/Logger.java b/testng-core-api/src/main/java/org/testng/log4testng/Logger.java
index 7dad29acaa..adb21922d4 100644
--- a/testng-core-api/src/main/java/org/testng/log4testng/Logger.java
+++ b/testng-core-api/src/main/java/org/testng/log4testng/Logger.java
@@ -1,6 +1,7 @@
 package org.testng.log4testng;
 
 import java.util.Map;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.slf4j.LoggerFactory;
 import org.testng.collections.Maps;
 
@@ -48,7 +49,7 @@ public boolean isTraceEnabled() {
    *
    * @param message the message object to log.
    */
-  public void trace(Object message) {
+  public void trace(@Nullable Object message) {
     logger.trace("{}", message);
   }
 
@@ -59,7 +60,7 @@ public void trace(Object message) {
    * @param message the message object to log.
    * @param t the exception to log, including its stack trace.
    */
-  public void trace(Object message, Throwable t) {
+  public void trace(@Nullable Object message, Throwable t) {
     logger.trace("{}", message, t);
   }
 
@@ -78,7 +79,7 @@ public boolean isDebugEnabled() {
    *
    * @param message the message object to log.
    */
-  public void debug(Object message) {
+  public void debug(@Nullable Object message) {
     logger.debug("{}", message);
   }
 
@@ -89,7 +90,7 @@ public void debug(Object message) {
    * @param message the message object to log.
    * @param t the exception to log, including its stack trace.
    */
-  public void debug(Object message, Throwable t) {
+  public void debug(@Nullable Object message, Throwable t) {
     logger.debug("{}", message, t);
   }
 
@@ -108,7 +109,7 @@ public boolean isInfoEnabled() {
    *
    * @param message the message object to log.
    */
-  public void info(Object message) {
+  public void info(@Nullable Object message) {
     logger.info("{}", message);
   }
 
@@ -119,7 +120,7 @@ public void info(Object message) {
    * @param message the message object to log.
    * @param t the exception to log, including its stack trace.
    */
-  public void info(Object message, Throwable t) {
+  public void info(@Nullable Object message, Throwable t) {
     logger.info("{}", message, t);
   }
 
@@ -129,7 +130,7 @@ public void info(Object message, Throwable t) {
    *
    * @param message the message object to log.
    */
-  public void warn(Object message) {
+  public void warn(@Nullable Object message) {
     logger.warn("{}", message);
   }
 
@@ -140,7 +141,7 @@ public void warn(Object message) {
    * @param message the message object to log.
    * @param t the exception to log, including its stack trace.
    */
-  public void warn(Object message, Throwable t) {
+  public void warn(@Nullable Object message, Throwable t) {
     logger.warn("{}", message, t);
   }
 
@@ -150,7 +151,7 @@ public void warn(Object message, Throwable t) {
    *
    * @param message the message object to log.
    */
-  public void error(Object message) {
+  public void error(@Nullable Object message) {
     logger.error("{}", message);
   }
 
@@ -161,7 +162,7 @@ public void error(Object message) {
    * @param message the message object to log.
    * @param t the exception to log, including its stack trace.
    */
-  public void error(Object message, Throwable t) {
+  public void error(@Nullable Object message, Throwable t) {
     logger.error("{}", message, t);
   }
 
@@ -171,7 +172,7 @@ public void error(Object message, Throwable t) {
    *
    * @param message the message object to log.
    */
-  public void fatal(Object message) {
+  public void fatal(@Nullable Object message) {
     logger.error("{}", message);
   }
 
@@ -182,7 +183,7 @@ public void fatal(Object message) {
    * @param message the message object to log.
    * @param t the exception to log, including its stack trace.
    */
-  public void fatal(Object message, Throwable t) {
+  public void fatal(@Nullable Object message, Throwable t) {
     logger.error("{}", message, t);
   }
 
diff --git a/testng-core-api/src/main/java/org/testng/package-info.java b/testng-core-api/src/main/java/org/testng/package-info.java
new file mode 100644
index 0000000000..bf86fddb16
--- /dev/null
+++ b/testng-core-api/src/main/java/org/testng/package-info.java
@@ -0,0 +1,6 @@
+@DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.ALL)
+package org.testng;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.framework.qual.DefaultQualifier;
+import org.checkerframework.framework.qual.TypeUseLocation;
diff --git a/testng-core-api/src/main/java/org/testng/reporters/XMLStringBuffer.java b/testng-core-api/src/main/java/org/testng/reporters/XMLStringBuffer.java
index 259637579c..5f0b7f6b9c 100644
--- a/testng-core-api/src/main/java/org/testng/reporters/XMLStringBuffer.java
+++ b/testng-core-api/src/main/java/org/testng/reporters/XMLStringBuffer.java
@@ -28,7 +28,7 @@ public class XMLStringBuffer {
   /** A string of space character representing the current indentation. */
   private String m_currentIndent = "";
 
-  private String defaultComment = null;
+  private @Nullable String defaultComment = null;
 
   public XMLStringBuffer() {
     init(Buffer.create(), "", "1.0", "UTF-8");
@@ -61,7 +61,7 @@ private void init(
       IBuffer buffer, String start, @Nullable String version, @Nullable String encoding) {
     m_buffer = buffer;
     m_currentIndent = start;
-    if (version != null) {
+    if (version != null && encoding != null) {
       setXmlDetails(version, encoding);
     }
   }
@@ -160,7 +160,7 @@ public void pop() {
    *
    * @param tagName The name of the tag this pop() is supposed to match.
    */
-  public void pop(String tagName) {
+  public void pop(@Nullable String tagName) {
     m_currentIndent = m_currentIndent.substring(DEFAULT_INDENT_INCREMENT.length());
     Tag t = m_tagStack.pop();
     if (null != tagName) {
@@ -171,7 +171,7 @@ public void pop(String tagName) {
       }
     }
 
-    String comment = defaultComment;
+    @Nullable String comment = defaultComment;
     if (comment == null) {
       comment = XMLUtils.extractComment(tagName, t.properties);
     }
@@ -293,11 +293,11 @@ public void addString(String s) {
     m_buffer.append(s);
   }
 
-  public void setDefaultComment(String defaultComment) {
+  public void setDefaultComment(@Nullable String defaultComment) {
     this.defaultComment = defaultComment;
   }
 
-  public void addCDATA(String content) {
+  public void addCDATA(@Nullable String content) {
     if (content != null) {
       // Solution from https://coderanch.com/t/455930/java/Remove-control-characters
       content = content.replaceAll("[\\p{Cc}&&[^\\r\\n]]", "");
@@ -352,9 +352,9 @@ public void toWriter(Writer fw) {
 class Tag {
   public final String tagName;
   public final String indent;
-  public final Properties properties;
+  public final @Nullable Properties properties;
 
-  public Tag(String ind, String n, Properties p) {
+  public Tag(String ind, String n, @Nullable Properties p) {
     tagName = n;
     indent = ind;
     properties = p;
diff --git a/testng-core-api/src/main/java/org/testng/reporters/XMLUtils.java b/testng-core-api/src/main/java/org/testng/reporters/XMLUtils.java
index 6f29acdc64..8dbefd0d05 100644
--- a/testng-core-api/src/main/java/org/testng/reporters/XMLUtils.java
+++ b/testng-core-api/src/main/java/org/testng/reporters/XMLUtils.java
@@ -42,7 +42,8 @@ public static String xml(
     return result.toString();
   }
 
-  public static String extractComment(String tag, Properties properties) {
+  public static @Nullable String extractComment(
+      @Nullable String tag, @Nullable Properties properties) {
     if (properties == null || "span".equals(tag)) return null;
 
     String[] attributes = new String[] {"id", "name", "class"};
@@ -68,7 +69,7 @@ public static void xmlOptional(
       String sp,
       String elementName,
       @Nullable String value,
-      Properties attributes) {
+      @Nullable Properties attributes) {
     if (null != value) {
       xmlRequired(result, sp, elementName, value, attributes);
     }
@@ -83,7 +84,8 @@ public static void xmlRequired(
     result.append(xml(sp, elementName, value, attributes));
   }
 
-  public static void xmlOpen(IBuffer result, String indent, String tag, Properties attributes) {
+  public static void xmlOpen(
+      IBuffer result, String indent, String tag, @Nullable Properties attributes) {
     xmlOpen(result, indent, tag, attributes, false /* no newline */);
   }
 
@@ -94,7 +96,7 @@ public static void xmlOpen(IBuffer result, String indent, String tag, Properties
    * @param result the buffer to append attributes to.
    * @param attributes the attributes to append (may be null).
    */
-  public static void appendAttributes(IBuffer result, Properties attributes) {
+  public static void appendAttributes(IBuffer result, @Nullable Properties attributes) {
     if (null != attributes) {
       for (Object element : attributes.entrySet()) {
         Entry entry = (Entry) element;
@@ -106,7 +108,11 @@ public static void appendAttributes(IBuffer result, Properties attributes) {
   }
 
   public static void xmlOpen(
-      IBuffer result, String indent, String tag, Properties attributes, boolean noNewLine) {
+      IBuffer result,
+      String indent,
+      String tag,
+      @Nullable Properties attributes,
+      boolean noNewLine) {
     result.append(indent).append("<").append(tag);
     appendAttributes(result, attributes);
     result.append(">");
@@ -115,7 +121,7 @@ public static void xmlOpen(
     }
   }
 
-  public static void xmlClose(IBuffer result, String indent, String tag, String comment) {
+  public static void xmlClose(IBuffer result, String indent, String tag, @Nullable String comment) {
     result
         .append(indent)
         .append("</")
diff --git a/testng-core-api/src/main/java/org/testng/xml/DefaultXmlWeaver.java b/testng-core-api/src/main/java/org/testng/xml/DefaultXmlWeaver.java
index 5beb32a632..ed3fd874e3 100644
--- a/testng-core-api/src/main/java/org/testng/xml/DefaultXmlWeaver.java
+++ b/testng-core-api/src/main/java/org/testng/xml/DefaultXmlWeaver.java
@@ -7,6 +7,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Properties;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.reporters.XMLStringBuffer;
 
 /**
@@ -20,13 +21,13 @@ class DefaultXmlWeaver implements IWeaveXml {
 
   private static final String HTTPS_TESTNG_DTD_URL = "https://testng.org/" + TESTNG_DTD;
 
-  private final String defaultComment;
+  private final @Nullable String defaultComment;
 
   DefaultXmlWeaver() {
     this(null);
   }
 
-  DefaultXmlWeaver(String defaultComment) {
+  DefaultXmlWeaver(@Nullable String defaultComment) {
     this.defaultComment = defaultComment;
   }
 
diff --git a/testng-core-api/src/main/java/org/testng/xml/XmlClass.java b/testng-core-api/src/main/java/org/testng/xml/XmlClass.java
index 5ce678579f..57b8d482c7 100644
--- a/testng-core-api/src/main/java/org/testng/xml/XmlClass.java
+++ b/testng-core-api/src/main/java/org/testng/xml/XmlClass.java
@@ -3,6 +3,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Properties;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.TestNGException;
 import org.testng.collections.Lists;
 import org.testng.collections.Maps;
@@ -15,8 +16,8 @@ public class XmlClass implements Cloneable {
 
   private List<XmlInclude> m_includedMethods = Lists.newArrayList();
   private List<String> m_excludedMethods = Lists.newArrayList();
-  private String m_name = null;
-  private Class m_class = null;
+  private String m_name;
+  private @Nullable Class<?> m_class = null;
   /** The index of this class in the &lt;test&gt; tag */
   private int m_index;
   /** True if the classes need to be loaded */
@@ -26,58 +27,60 @@ public class XmlClass implements Cloneable {
   private XmlTest m_xmlTest;
 
   public XmlClass() {
-    init("", null, 0, false /* load classes */);
+    this("", null, 0, false /* load classes */);
   }
 
   public XmlClass(String name) {
-    init(name, null, 0);
+    this(name, null, 0);
   }
 
   public XmlClass(String name, boolean loadClasses) {
-    init(name, null, 0, loadClasses);
+    this(name, null, 0, loadClasses);
   }
 
-  public XmlClass(Class cls) {
-    init(cls.getName(), cls, 0, true);
+  public XmlClass(Class<?> cls) {
+    this(cls.getName(), cls, 0, true);
   }
 
-  public XmlClass(Class cls, boolean loadClasses) {
-    init(cls.getName(), cls, 0, loadClasses);
+  public XmlClass(Class<?> cls, boolean loadClasses) {
+    this(cls.getName(), cls, 0, loadClasses);
   }
 
   public XmlClass(String className, int index) {
-    init(className, null, index, true /* load classes */);
+    this(className, null, index, true /* load classes */);
   }
 
   public XmlClass(String className, int index, boolean loadClasses) {
-    init(className, null, index, loadClasses);
+    this(className, null, index, loadClasses);
   }
 
-  private void init(String className, Class cls, int index) {
-    init(className, cls, index, true /* load classes */);
+  private XmlClass(String className, @Nullable Class<?> cls, int index) {
+    this(className, cls, index, true /* load classes */);
   }
 
-  private void init(String className, Class cls, int index, boolean resolveClass) {
+  private XmlClass(String className, @Nullable Class<?> cls, int index, boolean resolveClass) {
     m_name = className;
-    m_class = cls;
-    m_index = index;
-
-    if (null == m_class && resolveClass) {
-      loadClass();
+    if (cls == null && resolveClass) {
+      m_class = loadClass(m_name);
+    } else {
+      m_class = null;
     }
+    m_index = index;
   }
 
-  private void loadClass() {
-    m_class = ClassHelper.forName(m_name);
-
-    if (null == m_class) {
-      throw new TestNGException("Cannot find class in classpath: " + m_name);
+  private static Class<?> loadClass(String name) {
+    @Nullable Class<?> clazz = ClassHelper.forName(name);
+    if (clazz == null) {
+      throw new TestNGException("Cannot find class in classpath: " + name);
     }
+    return clazz;
   }
 
   /** @return Returns the className. */
   public Class<?> getSupportClass() {
-    if (m_class == null) loadClass();
+    if (m_class == null) {
+      m_class = loadClass(m_name);
+    }
     return m_class;
   }
 
@@ -210,19 +213,19 @@ public int hashCode() {
   }
 
   @Override
-  public boolean equals(Object obj) {
+  public boolean equals(@Nullable Object obj) {
     if (this == obj) {
       return true;
     }
-    if (obj == null) return XmlSuite.f();
-    if (getClass() != obj.getClass()) return XmlSuite.f();
+    if (obj == null) return false;
+    if (getClass() != obj.getClass()) return false;
     XmlClass other = (XmlClass) obj;
     if (other.m_loadClasses != m_loadClasses) {
-      return XmlSuite.f();
+      return false;
     }
     if (m_name == null) {
-      if (other.m_name != null) return XmlSuite.f();
-    } else if (!m_name.equals(other.m_name)) return XmlSuite.f();
+      if (other.m_name != null) return false;
+    } else if (!m_name.equals(other.m_name)) return false;
 
     return true;
   }
diff --git a/testng-core-api/src/main/java/org/testng/xml/XmlDefine.java b/testng-core-api/src/main/java/org/testng/xml/XmlDefine.java
index ea74ff06ef..8dd236a58a 100644
--- a/testng-core-api/src/main/java/org/testng/xml/XmlDefine.java
+++ b/testng-core-api/src/main/java/org/testng/xml/XmlDefine.java
@@ -3,6 +3,7 @@
 import static org.testng.collections.CollectionUtils.hasElements;
 
 import java.util.List;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.collections.Lists;
 import org.testng.reporters.XMLStringBuffer;
 
@@ -45,7 +46,7 @@ public List<String> getIncludes() {
   }
 
   @Override
-  public boolean equals(Object o) {
+  public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/testng-core-api/src/main/java/org/testng/xml/XmlInclude.java b/testng-core-api/src/main/java/org/testng/xml/XmlInclude.java
index 289bdfc31e..bad68c4fc2 100644
--- a/testng-core-api/src/main/java/org/testng/xml/XmlInclude.java
+++ b/testng-core-api/src/main/java/org/testng/xml/XmlInclude.java
@@ -6,6 +6,7 @@
 import java.util.Map;
 import java.util.Properties;
 import java.util.Set;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.collections.Lists;
 import org.testng.collections.Maps;
 import org.testng.reporters.XMLStringBuffer;
@@ -103,21 +104,21 @@ public int hashCode() {
   }
 
   @Override
-  public boolean equals(Object obj) {
+  public boolean equals(@Nullable Object obj) {
     if (this == obj) return true;
-    if (obj == null) return XmlSuite.f();
-    if (getClass() != obj.getClass()) return XmlSuite.f();
+    if (obj == null) return false;
+    if (getClass() != obj.getClass()) return false;
     XmlInclude other = (XmlInclude) obj;
     // if (m_index != other.m_index)
-    // return XmlSuite.f();
+    // return false;
     if (m_invocationNumbers == null) {
-      if (other.m_invocationNumbers != null) return XmlSuite.f();
-    } else if (!m_invocationNumbers.equals(other.m_invocationNumbers)) return XmlSuite.f();
+      if (other.m_invocationNumbers != null) return false;
+    } else if (!m_invocationNumbers.equals(other.m_invocationNumbers)) return false;
     if (m_name == null) {
-      if (other.m_name != null) return XmlSuite.f();
-    } else if (!m_name.equals(other.m_name)) return XmlSuite.f();
+      if (other.m_name != null) return false;
+    } else if (!m_name.equals(other.m_name)) return false;
     if (!m_parameters.equals(other.m_parameters)) {
-      return XmlSuite.f();
+      return false;
     }
     return true;
   }
diff --git a/testng-core-api/src/main/java/org/testng/xml/XmlMethodSelector.java b/testng-core-api/src/main/java/org/testng/xml/XmlMethodSelector.java
index be7ac5feb6..010616a134 100644
--- a/testng-core-api/src/main/java/org/testng/xml/XmlMethodSelector.java
+++ b/testng-core-api/src/main/java/org/testng/xml/XmlMethodSelector.java
@@ -1,42 +1,43 @@
 package org.testng.xml;
 
 import java.util.Properties;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.TestNGException;
 import org.testng.reporters.XMLStringBuffer;
 
 /** This class describes the tag <code>&lt;method-selector&gt;</code> in testng.xml. */
 public class XmlMethodSelector {
   // Either this:
-  private String m_className;
+  private @Nullable String m_className;
   private int m_priority;
 
   // Or that:
-  private XmlScript m_script;
+  private @Nullable XmlScript m_script;
 
   // For YAML
-  public void setClassName(String s) {
+  public void setClassName(@Nullable String s) {
     m_className = s;
   }
 
-  public String getClassName() {
+  public @Nullable String getClassName() {
     return m_className;
   }
 
   // For YAML
-  public void setElement(String name, String priority) {
+  public void setElement(@Nullable String name, String priority) {
     setName(name);
     setPriority(Integer.parseInt(priority));
   }
 
-  public void setName(String name) {
+  public void setName(@Nullable String name) {
     m_className = name;
   }
 
-  public XmlScript getScript() {
+  public @Nullable XmlScript getScript() {
     return m_script;
   }
 
-  public void setScript(XmlScript script) {
+  public void setScript(@Nullable XmlScript script) {
     m_script = script;
   }
 
@@ -55,19 +56,26 @@ public String toXml(String indent) {
 
     if (null != m_className) {
       Properties clsProp = new Properties();
-      clsProp.setProperty("name", getClassName());
+      clsProp.setProperty("name", m_className);
       if (getPriority() != -1) {
         clsProp.setProperty("priority", String.valueOf(getPriority()));
       }
       xsb.addEmptyElement("selector-class", clsProp);
-    } else if (getScript() != null && getScript().getLanguage() != null) {
+    } else {
+      @Nullable XmlScript script = getScript();
+      if (script == null) {
+        throw new TestNGException("Invalid Method Selector: found neither class name nor script");
+      }
+      @Nullable String language = script.getLanguage();
+      @Nullable String expression = script.getExpression();
+      if (language == null || expression == null) {
+        throw new TestNGException("Invalid Method Selector: found neither expression nor language");
+      }
       Properties scriptProp = new Properties();
-      scriptProp.setProperty("language", getScript().getLanguage());
+      scriptProp.setProperty("language", language);
       xsb.push("script", scriptProp);
-      xsb.addCDATA(getScript().getExpression());
+      xsb.addCDATA(expression);
       xsb.pop("script");
-    } else {
-      throw new TestNGException("Invalid Method Selector:  found neither class name nor language");
     }
 
     xsb.pop("method-selector");
@@ -80,45 +88,43 @@ public int hashCode() {
     final int prime = 31;
     int result = 1;
     result = prime * result + ((m_className == null) ? 0 : m_className.hashCode());
-    if (getScript() != null) {
-      result =
-          prime * result
-              + ((getScript().getExpression() == null)
-                  ? 0
-                  : getScript().getExpression().hashCode());
-      result =
-          prime * result
-              + ((getScript().getLanguage() == null) ? 0 : getScript().getLanguage().hashCode());
+    @Nullable XmlScript script = getScript();
+    if (script != null) {
+      @Nullable String expression = script.getExpression();
+      @Nullable String language = script.getLanguage();
+      result = prime * result + ((expression == null) ? 0 : expression.hashCode());
+      result = prime * result + ((language == null) ? 0 : language.hashCode());
     }
     result = prime * result + m_priority;
     return result;
   }
 
   @Override
-  public boolean equals(Object obj) {
+  public boolean equals(@Nullable Object obj) {
     if (this == obj) return true;
-    if (obj == null) return XmlSuite.f();
-    if (getClass() != obj.getClass()) return XmlSuite.f();
+    if (obj == null) return false;
+    if (getClass() != obj.getClass()) return false;
     XmlMethodSelector other = (XmlMethodSelector) obj;
     if (m_className == null) {
-      if (other.m_className != null) return XmlSuite.f();
-    } else if (!m_className.equals(other.m_className)) return XmlSuite.f();
-    if (getScript() == null || getScript().getExpression() == null) {
-      if (other.getScript() != null && other.getScript().getExpression() != null)
-        return XmlSuite.f();
-    } else if (!getScript()
+      if (other.m_className != null) return false;
+    } else if (!m_className.equals(other.m_className)) return false;
+    @Nullable XmlScript script = getScript();
+    @Nullable XmlScript otherScript = other.getScript();
+    if (script == null || script.getExpression() == null) {
+      if (otherScript != null && otherScript.getExpression() != null) return false;
+    } else if (!script
         .getExpression()
-        .equals(other.getScript() == null ? null : other.getScript().getExpression())) {
-      return XmlSuite.f();
+        .equals(otherScript == null ? null : otherScript.getExpression())) {
+      return false;
     }
-    if (getScript() == null || getScript().getLanguage() == null) {
-      if (other.getScript() != null && other.getScript().getLanguage() != null) return XmlSuite.f();
-    } else if (!getScript()
+    if (script == null || script.getLanguage() == null) {
+      if (otherScript != null && otherScript.getLanguage() != null) return false;
+    } else if (!script
         .getLanguage()
-        .equals(other.getScript() == null ? null : other.getScript().getLanguage())) {
-      return XmlSuite.f();
+        .equals(otherScript == null ? null : otherScript.getLanguage())) {
+      return false;
     }
-    if (m_priority != other.m_priority) return XmlSuite.f();
+    if (m_priority != other.m_priority) return false;
     return true;
   }
 }
diff --git a/testng-core-api/src/main/java/org/testng/xml/XmlPackage.java b/testng-core-api/src/main/java/org/testng/xml/XmlPackage.java
index db5a5c2e8b..ee436c1275 100644
--- a/testng-core-api/src/main/java/org/testng/xml/XmlPackage.java
+++ b/testng-core-api/src/main/java/org/testng/xml/XmlPackage.java
@@ -3,6 +3,7 @@
 import java.io.IOException;
 import java.util.List;
 import java.util.Properties;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.collections.Lists;
 import org.testng.internal.PackageUtils;
 import org.testng.internal.Utils;
@@ -15,11 +16,12 @@ public class XmlPackage {
   private String m_name;
   private List<String> m_include = Lists.newArrayList();
   private List<String> m_exclude = Lists.newArrayList();
-  private List<XmlClass> m_xmlClasses = null;
+  private @Nullable List<XmlClass> m_xmlClasses = null;
 
+  @SuppressWarnings("initialization.fields.uninitialized")
+  // For YAML
   public XmlPackage() {}
 
-  // For YAML
   public XmlPackage(String name) {
     m_name = name;
   }
@@ -117,23 +119,23 @@ public int hashCode() {
   }
 
   @Override
-  public boolean equals(Object obj) {
+  public boolean equals(@Nullable Object obj) {
     if (this == obj) return true;
-    if (obj == null) return XmlSuite.f();
-    if (getClass() != obj.getClass()) return XmlSuite.f();
+    if (obj == null) return false;
+    if (getClass() != obj.getClass()) return false;
     XmlPackage other = (XmlPackage) obj;
     if (m_exclude == null) {
-      if (other.m_exclude != null) return XmlSuite.f();
-    } else if (!m_exclude.equals(other.m_exclude)) return XmlSuite.f();
+      if (other.m_exclude != null) return false;
+    } else if (!m_exclude.equals(other.m_exclude)) return false;
     if (m_include == null) {
-      if (other.m_include != null) return XmlSuite.f();
-    } else if (!m_include.equals(other.m_include)) return XmlSuite.f();
+      if (other.m_include != null) return false;
+    } else if (!m_include.equals(other.m_include)) return false;
     if (m_name == null) {
-      if (other.m_name != null) return XmlSuite.f();
-    } else if (!m_name.equals(other.m_name)) return XmlSuite.f();
+      if (other.m_name != null) return false;
+    } else if (!m_name.equals(other.m_name)) return false;
     if (m_xmlClasses == null) {
-      if (other.m_xmlClasses != null) return XmlSuite.f();
-    } else if (!m_xmlClasses.equals(other.m_xmlClasses)) return XmlSuite.f();
+      if (other.m_xmlClasses != null) return false;
+    } else if (!m_xmlClasses.equals(other.m_xmlClasses)) return false;
     return true;
   }
 }
diff --git a/testng-core-api/src/main/java/org/testng/xml/XmlScript.java b/testng-core-api/src/main/java/org/testng/xml/XmlScript.java
index 63d072f672..0f94f04fc1 100644
--- a/testng-core-api/src/main/java/org/testng/xml/XmlScript.java
+++ b/testng-core-api/src/main/java/org/testng/xml/XmlScript.java
@@ -1,9 +1,11 @@
 package org.testng.xml;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 public class XmlScript {
 
-  private String language;
-  private String expression;
+  private @Nullable String language = null;
+  private @Nullable String expression = null;
 
   public void setLanguage(String language) {
     this.language = language;
@@ -13,11 +15,11 @@ public void setExpression(String expression) {
     this.expression = expression;
   }
 
-  public String getExpression() {
+  public @Nullable String getExpression() {
     return expression;
   }
 
-  public String getLanguage() {
+  public @Nullable String getLanguage() {
     return language;
   }
 }
diff --git a/testng-core-api/src/main/java/org/testng/xml/XmlSuite.java b/testng-core-api/src/main/java/org/testng/xml/XmlSuite.java
index f8a11b936f..a4c8ada59c 100644
--- a/testng-core-api/src/main/java/org/testng/xml/XmlSuite.java
+++ b/testng-core-api/src/main/java/org/testng/xml/XmlSuite.java
@@ -4,7 +4,7 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
-import java.util.Set;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.ITestObjectFactory;
 import org.testng.collections.Lists;
 import org.testng.collections.Maps;
@@ -86,7 +86,7 @@ public enum FailurePolicy {
       this.name = name;
     }
 
-    public static FailurePolicy getValidPolicy(String policy) {
+    public static @Nullable FailurePolicy getValidPolicy(String policy) {
       if (policy == null) {
         return null;
       }
@@ -103,8 +103,6 @@ public String toString() {
     }
   }
 
-  private String m_test;
-
   /** The default suite name TODO CQ is this OK as a default name. */
   private static final String DEFAULT_SUITE_NAME = "Default Suite";
 
@@ -114,7 +112,7 @@ public String toString() {
   /** The suite verbose flag (0 to 10). */
   public static final Integer DEFAULT_VERBOSE = 1;
 
-  private Integer m_verbose = null;
+  private @Nullable Integer m_verbose = null;
 
   public static final ParallelMode DEFAULT_PARALLEL = ParallelMode.NONE;
   private ParallelMode m_parallel = DEFAULT_PARALLEL;
@@ -169,16 +167,16 @@ public String toString() {
   private Map<String, String> m_parameters = Maps.newHashMap();
 
   /** Name of the XML file. */
-  private String m_fileName;
+  private @Nullable String m_fileName;
 
   /** Time out for methods/tests. */
-  private String m_timeOut;
+  private @Nullable String m_timeOut;
 
   /** List of child XML suites specified using <suite-file> tags. */
   private final List<XmlSuite> m_childSuites = Lists.newArrayList();
 
   /** Parent XML suite if this suite was specified in another suite using <suite-file> tag. */
-  private XmlSuite m_parentSuite;
+  private @Nullable XmlSuite m_parentSuite;
 
   private List<String> m_suiteFiles = Lists.newArrayList();
 
@@ -204,12 +202,12 @@ public boolean isParsed() {
   }
 
   /** @return The fileName. */
-  public String getFileName() {
+  public @Nullable String getFileName() {
     return m_fileName;
   }
 
   /** @param fileName The fileName to set. */
-  public void setFileName(String fileName) {
+  public void setFileName(@Nullable String fileName) {
     m_fileName = fileName;
   }
 
@@ -328,12 +326,13 @@ public void setName(String name) {
   }
 
   /**
-   * Returns the test.
-   *
+   * @deprecated Returns the test.
    * @return The test.
    */
+  @Deprecated
   public String getTest() {
-    return m_test;
+    throw new UnsupportedOperationException(
+        "getTest() is not supported anymore. Please use getTests() instead.");
   }
 
   /**
@@ -386,11 +385,9 @@ private void updateParameters() {
      * suite override the same named parameters from the parent suite.
      */
     if (m_parentSuite != null) {
-      Set<String> keySet = m_parentSuite.getParameters().keySet();
-      for (String name : keySet) {
-        if (!m_parameters.containsKey(name)) {
-          m_parameters.put(name, m_parentSuite.getParameter(name));
-        }
+      Map<String, String> parentParameters = m_parentSuite.getParameters();
+      for (Map.Entry<String, String> parentParameter : parentParameters.entrySet()) {
+        m_parameters.putIfAbsent(parentParameter.getKey(), parentParameter.getValue());
       }
     }
   }
@@ -432,7 +429,7 @@ public Map<String, String> getAllParameters() {
    * @param name The parameter name.
    * @return The parameter defined in this suite only.
    */
-  public String getParameter(String name) {
+  public @Nullable String getParameter(String name) {
     return m_parameters.get(name);
   }
 
@@ -599,7 +596,7 @@ public XmlSuite shallowCopy() {
    *
    * @param timeOut The timeout.
    */
-  public void setTimeOut(String timeOut) {
+  public void setTimeOut(@Nullable String timeOut) {
     m_timeOut = timeOut;
   }
 
@@ -608,7 +605,7 @@ public void setTimeOut(String timeOut) {
    *
    * @return The timeout.
    */
-  public String getTimeOut() {
+  public @Nullable String getTimeOut() {
     return m_timeOut;
   }
 
@@ -671,12 +668,12 @@ public int getDataProviderThreadCount() {
     return m_dataProviderThreadCount;
   }
 
-  public void setParentSuite(XmlSuite parentSuite) {
+  public void setParentSuite(@Nullable XmlSuite parentSuite) {
     m_parentSuite = parentSuite;
     updateParameters();
   }
 
-  public XmlSuite getParentSuite() {
+  public @Nullable XmlSuite getParentSuite() {
     return m_parentSuite;
   }
 
@@ -710,7 +707,6 @@ public int hashCode() {
                 ? 0
                 : m_skipFailedInvocationCounts.hashCode());
     result = prime * result + ((m_suiteFiles == null) ? 0 : m_suiteFiles.hashCode());
-    result = prime * result + ((m_test == null) ? 0 : m_test.hashCode());
     result = prime * result + ((m_tests == null) ? 0 : m_tests.hashCode());
     result = prime * result + m_threadCount;
     result = prime * result + ((m_timeOut == null) ? 0 : m_timeOut.hashCode());
@@ -719,114 +715,107 @@ public int hashCode() {
     return result;
   }
 
-  /** Used to debug equals() bugs. */
-  static boolean f() {
-    return false;
-  }
-
   @Override
-  public boolean equals(Object obj) {
+  public boolean equals(@Nullable Object obj) {
     if (this == obj) {
       return true;
     }
     if (obj == null) {
-      return f();
+      return false;
     }
     if (getClass() != obj.getClass()) {
-      return f();
+      return false;
     }
     XmlSuite other = (XmlSuite) obj;
     //      if (m_childSuites == null) {
     //        if (other.m_childSuites != null)
-    //          return f();
+    //          return false;
     //      } else if (!m_childSuites.equals(other.m_childSuites))
-    //        return f();
+    //        return false;
     if (m_configFailurePolicy == null) {
       if (other.m_configFailurePolicy != null) {
-        return f();
+        return false;
       }
     } else if (!m_configFailurePolicy.equals(other.m_configFailurePolicy)) {
-      return f();
+      return false;
     }
     if (m_dataProviderThreadCount != other.m_dataProviderThreadCount) {
-      return f();
+      return false;
     }
     if (m_isJUnit == null) {
       if (other.m_isJUnit != null) {
-        return f();
+        return false;
       }
     } else if (!m_isJUnit.equals(other.m_isJUnit)) {
-      return f();
+      return false;
     }
     if (m_listeners == null) {
       if (other.m_listeners != null) {
-        return f();
+        return false;
       }
     } else if (!m_listeners.equals(other.m_listeners)) {
-      return f();
+      return false;
     }
     if (m_methodSelectors == null) {
       if (other.m_methodSelectors != null) {
-        return f();
+        return false;
       }
     } else if (!m_methodSelectors.equals(other.m_methodSelectors)) {
-      return f();
+      return false;
     }
     if (m_name == null) {
       if (other.m_name != null) {
-        return f();
+        return false;
       }
     } else if (!m_name.equals(other.m_name)) {
-      return f();
+      return false;
     }
     if (m_objectFactoryClass == null) {
       if (other.m_objectFactoryClass != null) {
-        return f();
+        return false;
       }
     } else if (!m_objectFactoryClass.equals(other.m_objectFactoryClass)) {
-      return f();
+      return false;
     }
     if (m_parallel == null) {
       if (other.m_parallel != null) {
-        return f();
+        return false;
       }
     } else if (!m_parallel.equals(other.m_parallel)) {
-      return f();
+      return false;
     }
     //    if (m_parameters == null) {
     //      if (other.m_parameters != null) {
-    //        return f();
+    //        return false;
     //      }
     //    } else if (!m_parameters.equals(other.m_parameters)) {
-    //      return f();
+    //      return false;
     //    }
     //      if (m_parentSuite == null) {
     //        if (other.m_parentSuite != null)
-    //          return f();
+    //          return false;
     //      } else if (!m_parentSuite.equals(other.m_parentSuite))
-    //        return f();
+    //        return false;
     if (m_skipFailedInvocationCounts == null) {
-      if (other.m_skipFailedInvocationCounts != null) return f();
-    } else if (!m_skipFailedInvocationCounts.equals(other.m_skipFailedInvocationCounts)) return f();
+      if (other.m_skipFailedInvocationCounts != null) return false;
+    } else if (!m_skipFailedInvocationCounts.equals(other.m_skipFailedInvocationCounts))
+      return false;
     if (m_suiteFiles == null) {
-      if (other.m_suiteFiles != null) return f();
-    } else if (!m_suiteFiles.equals(other.m_suiteFiles)) return f();
-    if (m_test == null) {
-      if (other.m_test != null) return f();
-    } else if (!m_test.equals(other.m_test)) return f();
+      if (other.m_suiteFiles != null) return false;
+    } else if (!m_suiteFiles.equals(other.m_suiteFiles)) return false;
     if (m_tests == null) {
-      if (other.m_tests != null) return f();
-    } else if (!m_tests.equals(other.m_tests)) return f();
-    if (m_threadCount != other.m_threadCount) return f();
+      if (other.m_tests != null) return false;
+    } else if (!m_tests.equals(other.m_tests)) return false;
+    if (m_threadCount != other.m_threadCount) return false;
     if (m_timeOut == null) {
-      if (other.m_timeOut != null) return f();
-    } else if (!m_timeOut.equals(other.m_timeOut)) return f();
+      if (other.m_timeOut != null) return false;
+    } else if (!m_timeOut.equals(other.m_timeOut)) return false;
     if (m_verbose == null) {
-      if (other.m_verbose != null) return f();
-    } else if (!m_verbose.equals(other.m_verbose)) return f();
+      if (other.m_verbose != null) return false;
+    } else if (!m_verbose.equals(other.m_verbose)) return false;
     if (m_xmlPackages == null) {
-      if (other.m_xmlPackages != null) return f();
-    } else if (!m_xmlPackages.equals(other.m_xmlPackages)) return f();
+      if (other.m_xmlPackages != null) return false;
+    } else if (!m_xmlPackages.equals(other.m_xmlPackages)) return false;
     return true;
   }
 
diff --git a/testng-core-api/src/main/java/org/testng/xml/XmlTest.java b/testng-core-api/src/main/java/org/testng/xml/XmlTest.java
index e408e0f327..c8f21d3e8d 100644
--- a/testng-core-api/src/main/java/org/testng/xml/XmlTest.java
+++ b/testng-core-api/src/main/java/org/testng/xml/XmlTest.java
@@ -1,6 +1,7 @@
 package org.testng.xml;
 
 import java.util.*;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.TestNGException;
 import org.testng.collections.Lists;
 import org.testng.collections.Maps;
@@ -19,7 +20,7 @@ public class XmlTest implements Cloneable {
   private List<XmlClass> m_xmlClasses = Lists.newArrayList();
 
   private Map<String, String> m_parameters = Maps.newHashMap();
-  private XmlSuite.ParallelMode m_parallel;
+  private XmlSuite.ParallelMode m_parallel = null;
 
   private List<XmlMethodSelector> m_methodSelectors = Lists.newArrayList();
   // test level packages
@@ -27,15 +28,15 @@ public class XmlTest implements Cloneable {
 
   private String m_timeOut;
   private Boolean m_skipFailedInvocationCounts = XmlSuite.DEFAULT_SKIP_FAILED_INVOCATION_COUNTS;
-  private Map<String, List<Integer>> m_failedInvocationNumbers = null; // lazily initialized
+  private final Map<String, List<Integer>> m_failedInvocationNumbers = Maps.newHashMap();
 
   private Boolean m_preserveOrder = XmlSuite.DEFAULT_PRESERVE_ORDER;
 
   private int m_index;
 
-  private Boolean m_groupByInstances;
+  private @Nullable Boolean m_groupByInstances;
 
-  private Boolean m_allowReturnValues = null;
+  private @Nullable Boolean m_allowReturnValues = null;
 
   private Map<String, String> m_xmlDependencyGroups = Maps.newHashMap();
 
@@ -46,14 +47,6 @@ public class XmlTest implements Cloneable {
    * @param index the index of this test tag in testng.xml
    */
   public XmlTest(XmlSuite suite, int index) {
-    init(suite, index);
-  }
-
-  public XmlTest(XmlSuite suite) {
-    init(suite, 0);
-  }
-
-  private void init(XmlSuite suite, int index) {
     m_suite = suite;
     m_suite.getTests().add(this);
     m_index = index;
@@ -62,7 +55,12 @@ private void init(XmlSuite suite, int index) {
     m_name = "Default XmlTest name " + UUID.randomUUID();
   }
 
+  public XmlTest(XmlSuite suite) {
+    this(suite, 0);
+  }
+
   // For YAML
+  @SuppressWarnings("initialization.fields.uninitialized")
   public XmlTest() {}
 
   public void setXmlPackages(List<XmlPackage> packages) {
@@ -104,7 +102,7 @@ public final XmlSuite getSuite() {
     return m_suite;
   }
 
-  /** @return the includedGroups. */
+  /** Returns the includedGroups. */
   public List<String> getIncludedGroups() {
     List<String> result = Lists.newArrayList();
     if (m_xmlGroups != null && m_xmlGroups.getRun() != null) {
@@ -118,7 +116,7 @@ public boolean isGroupFilteringDisabled() {
     return getIncludedGroups().isEmpty() && getExcludedGroups().isEmpty();
   }
 
-  /** @return Returns the classes. */
+  /** Returns the classes. */
   public List<XmlClass> getXmlClasses() {
     return m_xmlClasses;
   }
@@ -142,7 +140,7 @@ public void setXmlClasses(List<XmlClass> classes) {
     m_xmlClasses = classes;
   }
 
-  /** @return Returns the name. */
+  /** Returns the name. */
   public String getName() {
     return m_name;
   }
@@ -216,7 +214,7 @@ public void addExcludedGroup(String g) {
     m_xmlGroups.getRun().getExcludes().add(g);
   }
 
-  /** @return Returns the verbose. */
+  /** Returns the verbose. */
   public int getVerbose() {
     Integer result = m_verbose;
     if (null == result || XmlSuite.DEFAULT_VERBOSE.equals(m_verbose)) {
@@ -231,7 +229,7 @@ public int getVerbose() {
   }
 
   public boolean getGroupByInstances() {
-    Boolean result = m_groupByInstances;
+    @Nullable Boolean result = m_groupByInstances;
     if (result == null || XmlSuite.DEFAULT_GROUP_BY_INSTANCES.equals(m_groupByInstances)) {
       result = getSuite().getGroupByInstances();
     }
@@ -270,7 +268,7 @@ public void setSkipFailedInvocationCounts(boolean skip) {
     m_skipFailedInvocationCounts = skip;
   }
 
-  /** @return Returns the isJUnit. */
+  /** Returns the isJUnit. */
   public boolean skipFailedInvocationCounts() {
     Boolean result = m_skipFailedInvocationCounts;
     if (null == result) {
@@ -301,7 +299,7 @@ public void setMetaGroups(Map<String, List<String>> metaGroups) {
     }
   }
 
-  /** @return Returns the metaGroups. */
+  /** Returns the metaGroups. */
   public Map<String, List<String>> getMetaGroups() {
     if (m_xmlGroups == null) {
       return Collections.emptyMap();
@@ -323,8 +321,8 @@ public void addParameter(String key, String value) {
     m_parameters.put(key, value);
   }
 
-  public String getParameter(String name) {
-    String result = m_parameters.get(name);
+  public @Nullable String getParameter(String name) {
+    @Nullable String result = m_parameters.get(name);
     if (null == result) {
       result = getSuite().getParameter(name);
     }
@@ -332,7 +330,7 @@ public String getParameter(String name) {
     return result;
   }
 
-  /** @return the parameters defined in this test tag and the tags above it. */
+  /** Returns the parameters defined in this test tag and the tags above it. */
   public Map<String, String> getAllParameters() {
     Map<String, String> result = Maps.newHashMap();
     result.putAll(getSuite().getParameters());
@@ -341,8 +339,8 @@ public Map<String, String> getAllParameters() {
   }
 
   /**
-   * @return the parameters defined in this tag, and only this test tag. To retrieve the inherited
-   *     parameters as well, call {@code getAllParameters()}.
+   * Returns the parameters defined in this tag, and only this test tag. To retrieve the inherited
+   * parameters as well, call {@code getAllParameters()}.
    */
   public Map<String, String> getLocalParameters() {
     return m_parameters;
@@ -356,8 +354,8 @@ public XmlSuite.ParallelMode getParallel() {
     return Optional.ofNullable(m_parallel).orElse(getSuite().getParallel());
   }
 
-  public String getTimeOut() {
-    String result = getSuite().getTimeOut();
+  public @Nullable String getTimeOut() {
+    @Nullable String result = getSuite().getTimeOut();
     if (null != m_timeOut) {
       result = m_timeOut;
     }
@@ -367,8 +365,9 @@ public String getTimeOut() {
 
   public long getTimeOut(long def) {
     long result = def;
-    if (getTimeOut() != null) {
-      result = Long.parseLong(getTimeOut());
+    @Nullable String timeOut = getTimeOut();
+    if (timeOut != null) {
+      result = Long.parseLong(timeOut);
     }
 
     return result;
@@ -378,7 +377,7 @@ public void setTimeOut(long timeOut) {
     m_timeOut = Long.toString(timeOut);
   }
 
-  private void setTimeOut(String timeOut) {
+  private void setTimeOut(@Nullable String timeOut) {
     m_timeOut = timeOut;
   }
 
@@ -394,7 +393,7 @@ public void setScript(XmlScript script) {
     }
   }
 
-  public XmlScript getScript() {
+  public @Nullable XmlScript getScript() {
     List<XmlMethodSelector> selectors = getMethodSelectors();
     if (selectors.isEmpty()) {
       return null;
@@ -440,6 +439,22 @@ public Object clone() {
     return result;
   }
 
+  private Map<String, List<Integer>> getFailedInvocationNumbers() {
+    if (!m_failedInvocationNumbers.isEmpty()) {
+      return m_failedInvocationNumbers;
+    }
+    for (XmlClass c : getXmlClasses()) {
+      for (XmlInclude xi : c.getIncludedMethods()) {
+        List<Integer> invocationNumbers = xi.getInvocationNumbers();
+        if (!invocationNumbers.isEmpty()) {
+          String methodName = c.getName() + "." + xi.getName();
+          m_failedInvocationNumbers.put(methodName, invocationNumbers);
+        }
+      }
+    }
+    return m_failedInvocationNumbers;
+  }
+
   /**
    * Convenience method to cache the ordering numbers for methods.
    *
@@ -447,20 +462,7 @@ public Object clone() {
    * @return The invocation numbers of the method
    */
   public List<Integer> getInvocationNumbers(String method) {
-    if (m_failedInvocationNumbers == null) {
-      m_failedInvocationNumbers = Maps.newHashMap();
-      for (XmlClass c : getXmlClasses()) {
-        for (XmlInclude xi : c.getIncludedMethods()) {
-          List<Integer> invocationNumbers = xi.getInvocationNumbers();
-          if (invocationNumbers.size() > 0) {
-            String methodName = c.getName() + "." + xi.getName();
-            m_failedInvocationNumbers.put(methodName, invocationNumbers);
-          }
-        }
-      }
-    }
-
-    List<Integer> result = m_failedInvocationNumbers.get(method);
+    @Nullable List<Integer> result = getFailedInvocationNumbers().get(method);
     if (result == null) {
       // Don't use emptyList here since this list might end up receiving values if
       // the test run fails.
@@ -548,15 +550,15 @@ public int hashCode() {
   }
 
   @Override
-  public boolean equals(Object obj) {
+  public boolean equals(@Nullable Object obj) {
     if (this == obj) {
       return true;
     }
-    if (obj == null) return XmlSuite.f();
-    if (getClass() != obj.getClass()) return XmlSuite.f();
+    if (obj == null) return false;
+    if (getClass() != obj.getClass()) return false;
     XmlTest other = (XmlTest) obj;
     if (m_xmlGroups == null) {
-      if (other.m_xmlGroups != null) return XmlSuite.f();
+      if (other.m_xmlGroups != null) return false;
     } else {
       if (other.m_xmlGroups == null) {
         return false;
@@ -566,55 +568,53 @@ public boolean equals(Object obj) {
         return false;
       }
       if (!m_xmlGroups.getRun().getExcludes().equals(other.m_xmlGroups.getRun().getExcludes())) {
-        return XmlSuite.f();
+        return false;
       }
       if (!m_xmlGroups.getRun().getIncludes().equals(other.m_xmlGroups.getRun().getIncludes())) {
-        return XmlSuite.f();
+        return false;
       }
       if (!m_xmlGroups.getDefines().equals(other.m_xmlGroups.getDefines())) {
         return false;
       }
     }
     if (m_failedInvocationNumbers == null) {
-      if (other.m_failedInvocationNumbers != null) return XmlSuite.f();
-    } else if (!m_failedInvocationNumbers.equals(other.m_failedInvocationNumbers))
-      return XmlSuite.f();
+      if (other.m_failedInvocationNumbers != null) return false;
+    } else if (!m_failedInvocationNumbers.equals(other.m_failedInvocationNumbers)) return false;
     if (m_isJUnit == null) {
-      if (other.m_isJUnit != null && !other.m_isJUnit.equals(XmlSuite.DEFAULT_JUNIT))
-        return XmlSuite.f();
-    } else if (!m_isJUnit.equals(other.m_isJUnit)) return XmlSuite.f();
+      if (other.m_isJUnit != null && !other.m_isJUnit.equals(XmlSuite.DEFAULT_JUNIT)) return false;
+    } else if (!m_isJUnit.equals(other.m_isJUnit)) return false;
     if (m_methodSelectors == null) {
-      if (other.m_methodSelectors != null) return XmlSuite.f();
-    } else if (!m_methodSelectors.equals(other.m_methodSelectors)) return XmlSuite.f();
+      if (other.m_methodSelectors != null) return false;
+    } else if (!m_methodSelectors.equals(other.m_methodSelectors)) return false;
     if (m_name == null) {
-      if (other.m_name != null) return XmlSuite.f();
-    } else if (!m_name.equals(other.m_name)) return XmlSuite.f();
+      if (other.m_name != null) return false;
+    } else if (!m_name.equals(other.m_name)) return false;
     if (m_parallel == null) {
-      if (other.m_parallel != null) return XmlSuite.f();
-    } else if (!m_parallel.equals(other.m_parallel)) return XmlSuite.f();
+      if (other.m_parallel != null) return false;
+    } else if (!m_parallel.equals(other.m_parallel)) return false;
     if (m_parameters == null) {
-      if (other.m_parameters != null) return XmlSuite.f();
-    } else if (!m_parameters.equals(other.m_parameters)) return XmlSuite.f();
+      if (other.m_parameters != null) return false;
+    } else if (!m_parameters.equals(other.m_parameters)) return false;
     if (m_preserveOrder == null) {
-      if (other.m_preserveOrder != null) return XmlSuite.f();
-    } else if (!m_preserveOrder.equals(other.m_preserveOrder)) return XmlSuite.f();
+      if (other.m_preserveOrder != null) return false;
+    } else if (!m_preserveOrder.equals(other.m_preserveOrder)) return false;
     if (m_skipFailedInvocationCounts == null) {
-      if (other.m_skipFailedInvocationCounts != null) return XmlSuite.f();
+      if (other.m_skipFailedInvocationCounts != null) return false;
     } else if (!m_skipFailedInvocationCounts.equals(other.m_skipFailedInvocationCounts))
-      return XmlSuite.f();
-    if (m_threadCount != other.m_threadCount) return XmlSuite.f();
+      return false;
+    if (m_threadCount != other.m_threadCount) return false;
     if (m_timeOut == null) {
-      if (other.m_timeOut != null) return XmlSuite.f();
-    } else if (!m_timeOut.equals(other.m_timeOut)) return XmlSuite.f();
+      if (other.m_timeOut != null) return false;
+    } else if (!m_timeOut.equals(other.m_timeOut)) return false;
     if (m_verbose == null) {
-      if (other.m_verbose != null) return XmlSuite.f();
-    } else if (!m_verbose.equals(other.m_verbose)) return XmlSuite.f();
+      if (other.m_verbose != null) return false;
+    } else if (!m_verbose.equals(other.m_verbose)) return false;
     if (m_xmlClasses == null) {
-      if (other.m_xmlClasses != null) return XmlSuite.f();
-    } else if (!m_xmlClasses.equals(other.m_xmlClasses)) return XmlSuite.f();
+      if (other.m_xmlClasses != null) return false;
+    } else if (!m_xmlClasses.equals(other.m_xmlClasses)) return false;
     if (m_xmlPackages == null) {
-      if (other.m_xmlPackages != null) return XmlSuite.f();
-    } else if (!m_xmlPackages.equals(other.m_xmlPackages)) return XmlSuite.f();
+      if (other.m_xmlPackages != null) return false;
+    } else if (!m_xmlPackages.equals(other.m_xmlPackages)) return false;
 
     return true;
   }
@@ -662,8 +662,9 @@ public XmlGroups getXmlGroups() {
   }
 
   /**
+   * Returns <code>true</code> if the current test's name matches with any of the given names.
+   *
    * @param names The list of names to check.
-   * @return <code>true</code> if the current test's name matches with any of the given names.
    */
   public boolean nameMatchesAny(List<String> names) {
     return names.contains(getName());
diff --git a/testng-core/src/main/java/org/testng/package-info.java b/testng-core/src/main/java/org/testng/package-info.java
new file mode 100644
index 0000000000..bf86fddb16
--- /dev/null
+++ b/testng-core/src/main/java/org/testng/package-info.java
@@ -0,0 +1,6 @@
+@DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.ALL)
+package org.testng;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.framework.qual.DefaultQualifier;
+import org.checkerframework.framework.qual.TypeUseLocation;
diff --git a/testng-reflection-utils/src/main/java/org/testng/internal/reflect/ReflectionHelper.java b/testng-reflection-utils/src/main/java/org/testng/internal/reflect/ReflectionHelper.java
index 7351044c0f..7524774257 100644
--- a/testng-reflection-utils/src/main/java/org/testng/internal/reflect/ReflectionHelper.java
+++ b/testng-reflection-utils/src/main/java/org/testng/internal/reflect/ReflectionHelper.java
@@ -3,18 +3,20 @@
 import java.lang.annotation.Annotation;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.HashSet;
-import java.util.LinkedList;
 import java.util.List;
 import java.util.Set;
 import java.util.stream.Collectors;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.testng.collections.Lists;
 
-public class ReflectionHelper {
+public final class ReflectionHelper {
+  private ReflectionHelper() {}
   /**
-   * @return An array of all locally declared methods or equivalent thereof (such as default methods
-   *     on Java 8 based interfaces that the given class implements).
+   * Returns an array of all locally declared methods or equivalent thereof (such as default methods
+   * on Java 8 based interfaces that the given class implements).
    */
   public static Method[] getLocalMethods(Class<?> clazz) {
     Method[] declaredMethods = excludingMain(clazz);
@@ -39,13 +41,13 @@ public static Method[] getLocalMethods(Class<?> clazz) {
   }
 
   /**
-   * @return An array of all locally declared methods or equivalent thereof (such as default methods
-   *     on Java 8 based interfaces that the given class implements) but excludes the <code>main()
+   * Returns an array of all locally declared methods or equivalent thereof (such as default methods
+   * on Java 8 based interfaces that the given class implements) but excludes the <code>main()
    *     </code> method alone.
    */
   public static Method[] excludingMain(Class<?> clazz) {
     Method[] declaredMethods = clazz.getDeclaredMethods();
-    List<Method> pruned = new LinkedList<>();
+    List<Method> pruned = new ArrayList<>();
     for (Method declaredMethod : declaredMethods) {
       if ("main".equals(declaredMethod.getName())
           && isStaticVoid(declaredMethod)
@@ -66,13 +68,13 @@ && acceptsStringArray(declaredMethod)) {
    * @param <T> - The annotation type
    * @return - Either the annotation if found (or) <code>null.</code>
    */
-  public static <T extends Annotation> T findAnnotation(
+  public static <T extends Annotation> @Nullable T findAnnotation(
       Class<?> typedTestClass, Class<T> annotation) {
     if (typedTestClass == null || annotation == null) {
       return null;
     }
-    T ignore = null;
-    Class<?> testClass = typedTestClass;
+    @Nullable T ignore = null;
+    @Nullable Class<?> testClass = typedTestClass;
 
     while (testClass != null && testClass != Object.class) {
       ignore = testClass.getAnnotation(annotation);
@@ -106,9 +108,10 @@ private static List<Method> getDefaultMethods(Class<?> clazz) {
 
   private static Set<Class<?>> getAllInterfaces(Class<?> clazz) {
     Set<Class<?>> result = new HashSet<>();
-    while (clazz != null && clazz != Object.class) {
-      result.addAll(Arrays.asList(clazz.getInterfaces()));
-      clazz = clazz.getSuperclass();
+    @Nullable Class<?> superClass = clazz;
+    while (superClass != null && superClass != Object.class) {
+      result.addAll(Arrays.asList(superClass.getInterfaces()));
+      superClass = superClass.getSuperclass();
     }
     return result;
   }
diff --git a/testng-reflection-utils/src/main/java/org/testng/internal/reflect/package-info.java b/testng-reflection-utils/src/main/java/org/testng/internal/reflect/package-info.java
new file mode 100644
index 0000000000..cf7cac4b26
--- /dev/null
+++ b/testng-reflection-utils/src/main/java/org/testng/internal/reflect/package-info.java
@@ -0,0 +1,6 @@
+@DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.ALL)
+package org.testng.internal.reflect;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.framework.qual.DefaultQualifier;
+import org.checkerframework.framework.qual.TypeUseLocation;
diff --git a/testng-reflection-utils/testng-reflection-utils-build.gradle.kts b/testng-reflection-utils/testng-reflection-utils-build.gradle.kts
index 17304ed1b6..a4f83cda8b 100644
--- a/testng-reflection-utils/testng-reflection-utils-build.gradle.kts
+++ b/testng-reflection-utils/testng-reflection-utils-build.gradle.kts
@@ -4,4 +4,5 @@ plugins {
 
 dependencies {
     implementation(projects.testngCollections)
+    implementation("org.checkerframework:checker-qual:3.36.0")
 }
diff --git a/testng-runner-api/src/main/java/org/testng/internal/package-info.java b/testng-runner-api/src/main/java/org/testng/internal/package-info.java
new file mode 100644
index 0000000000..b113aab45c
--- /dev/null
+++ b/testng-runner-api/src/main/java/org/testng/internal/package-info.java
@@ -0,0 +1,6 @@
+@DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.ALL)
+package org.testng.internal;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.framework.qual.DefaultQualifier;
+import org.checkerframework.framework.qual.TypeUseLocation;
diff --git a/testng-test-kit/src/main/java/testhelper/PerformanceUtils.java b/testng-test-kit/src/main/java/testhelper/PerformanceUtils.java
index 6a804f63dd..b057e22127 100644
--- a/testng-test-kit/src/main/java/testhelper/PerformanceUtils.java
+++ b/testng-test-kit/src/main/java/testhelper/PerformanceUtils.java
@@ -4,13 +4,14 @@
 import java.lang.management.ThreadMXBean;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 public class PerformanceUtils {
 
   private static final ThreadMXBean THREAD_MX_BEAN = ManagementFactory.getThreadMXBean();
-  private static final Method GET_THREAD_ALLOCATED_BYTES = initGetThreadAllocatedBytes();
+  private static final @Nullable Method GET_THREAD_ALLOCATED_BYTES = initGetThreadAllocatedBytes();
 
-  private static Method initGetThreadAllocatedBytes() {
+  private static @Nullable Method initGetThreadAllocatedBytes() {
     try {
       Method method = THREAD_MX_BEAN.getClass().getMethod("getThreadAllocatedBytes", long.class);
       method.setAccessible(true);
@@ -25,7 +26,7 @@ public static boolean canMeasureAllocatedMemory() {
     return GET_THREAD_ALLOCATED_BYTES != null;
   }
 
-  /** @return amount of memory (in bytes) allocated by current thread until now */
+  /** Returns amount of memory (in bytes) allocated by current thread until now. */
   public static long measureAllocatedMemory() {
     if (GET_THREAD_ALLOCATED_BYTES == null) {
       throw new IllegalStateException(
@@ -34,7 +35,12 @@ public static long measureAllocatedMemory() {
     long selfId = Thread.currentThread().getId();
 
     try {
-      return (long) GET_THREAD_ALLOCATED_BYTES.invoke(THREAD_MX_BEAN, selfId);
+      Object result = GET_THREAD_ALLOCATED_BYTES.invoke(THREAD_MX_BEAN, selfId);
+      if (!(result instanceof Long)) {
+        throw new IllegalStateException(
+            "Method getThreadAllocatedBytes(long) returns unexpected type: " + result);
+      }
+      return (Long) result;
     } catch (IllegalAccessException e) {
       throw new RuntimeException("Unable to call " + GET_THREAD_ALLOCATED_BYTES, e);
     } catch (InvocationTargetException e) {
diff --git a/testng-test-kit/testng-test-kit-build.gradle.kts b/testng-test-kit/testng-test-kit-build.gradle.kts
index 0438ec3bf1..65a3160e70 100644
--- a/testng-test-kit/testng-test-kit-build.gradle.kts
+++ b/testng-test-kit/testng-test-kit-build.gradle.kts
@@ -1,3 +1,7 @@
 plugins {
     id("testng.java-library")
 }
+
+dependencies {
+    implementation("org.checkerframework:checker-qual:3.36.0")
+}
diff --git a/versions.properties b/versions.properties
index 9a7016ba18..7e765dbdde 100644
--- a/versions.properties
+++ b/versions.properties
@@ -7,6 +7,8 @@
 #### suppress inspection "SpellCheckingInspection" for whole file
 #### suppress inspection "UnusedProperty" for whole file
 
+plugin.org.checkerframework=0.6.29
+
 plugin.com.github.vlsi.stage-vote-release=1.78
 
 version.ch.qos.logback..logback-classic=1.2.11