[HUDI-615]: Add Test cases for StringUtils.

hudi-common/src/main/java/org/apache/hudi/common/util/StringUtils.java

@@ -18,6 +18,8 @@
 
 package org.apache.hudi.common.util;
 
+import javax.annotation.Nullable;
+
 /**
  * Simple utility for operations on strings.
  */
@@ -67,4 +69,29 @@ public static String toHexString(byte[] bytes) {
   public static boolean isNullOrEmpty(String str) {
     return str == null || str.length() == 0;
   }
+
+
+  /**
+   * Returns the given string if it is non-null; the empty string otherwise.
+   *
+   * @param string the string to test and possibly return
+   * @return {@code string} itself if it is non-null; {@code ""} if it is null
+   */
+  public static String nullToEmpty(@Nullable String string) {
+    return string == null ? "" : string;
+  }
+
+  /**
+   * Returns the given string if it is nonempty; {@code null} otherwise.
+   *
+   * @param string the string to test and possibly return
+   * @return {@code string} itself if it is nonempty; {@code null} if it is empty or null
+   */
+  public static @Nullable String emptyToNull(@Nullable String string) {
+    return stringIsNullOrEmpty(string) ? null : string;
+  }
+
+  private static boolean stringIsNullOrEmpty(@Nullable String string) {
+    return string == null || string.isEmpty();
+  }
 }

hudi-common/src/test/java/org/apache/hudi/common/util/TestStringUtils.java

@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hudi.common.util;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertTrue;
+
+public class TestStringUtils {
+
+  private static final String[] STRINGS = {"This", "is", "a", "test"};
+
+  @Test
+  public void testStringJoinWithDelim() {
+    String joinedString = StringUtils.joinUsingDelim("-", STRINGS);
+    assertEquals(STRINGS.length, joinedString.split("-").length);
+  }
+
+  @Test
+  public void testStringJoin() {
+    assertNotEquals(null, StringUtils.join(""));
+    assertNotEquals(null, StringUtils.join(STRINGS));
+  }
+
+  @Test
+  public void testStringNullToEmpty() {
+    String str = "This is a test";
+    assertEquals(str, StringUtils.nullToEmpty(str));
+    assertEquals("", StringUtils.nullToEmpty(null));
+  }
+
+  @Test
+  public void testStringEmptyToNull() {
+    assertNull(StringUtils.emptyToNull(""));
+    assertEquals("Test String", StringUtils.emptyToNull("Test String"));
+  }
+
+  @Test
+  public void testStringNullOrEmpty() {
+    assertTrue(StringUtils.isNullOrEmpty(null));
+    assertTrue(StringUtils.isNullOrEmpty(""));
+    assertNotEquals(null, StringUtils.isNullOrEmpty("this is not empty"));
+    assertTrue(StringUtils.isNullOrEmpty(""));
+  }
+}