Extension of to stream converter method.

Examples of benefits:

- Kotlin Sequence support for @testfactory
- Kotlin Sequence support for @MethodSource
- Classes that expose an Iterator returning method,
can be converted to a stream.

Issue: #3376

I hereby agree to the terms of the JUnit Contributor License Agreement.

Author: Hans Zuidervaart

documentation/src/docs/asciidoc/user-guide/writing-tests.adoc

@@ -2480,7 +2480,7 @@ generated at runtime by a factory method that is annotated with `@TestFactory`.
 In contrast to `@Test` methods, a `@TestFactory` method is not itself a test case but
 rather a factory for test cases. Thus, a dynamic test is the product of a factory.
 Technically speaking, a `@TestFactory` method must return a single `DynamicNode` or a
-`Stream`, `Collection`, `Iterable`, `Iterator`, or array of `DynamicNode` instances.
+`Stream`, `Collection`, `Iterable`, `Iterator`, an `Iterator` providing class or array of `DynamicNode` instances.
 Instantiable subclasses of `DynamicNode` are `DynamicContainer` and `DynamicTest`.
 `DynamicContainer` instances are composed of a _display name_ and a list of dynamic child
 nodes, enabling the creation of arbitrarily nested hierarchies of dynamic nodes.

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/TestFactoryTestDescriptor.java

@@ -132,7 +132,7 @@ private Stream<DynamicNode> toDynamicNodeStream(Object testFactoryMethodResult)
 
 	private JUnitException invalidReturnTypeException(Throwable cause) {
 		String message = String.format(
-			"@TestFactory method [%s] must return a single %2$s or a Stream, Collection, Iterable, Iterator, or array of %2$s.",
+			"@TestFactory method [%s] must return a single %2$s or a Stream, Collection, Iterable, Iterator, Iterator-source or array of %2$s.",
 			getTestMethod().toGenericString(), DynamicNode.class.getName());
 		return new JUnitException(message, cause);
 	}

junit-platform-commons/src/main/java/org/junit/platform/commons/util/CollectionUtils.java

@@ -18,6 +18,7 @@
 import static org.apiguardian.api.API.Status.INTERNAL;
 
 import java.lang.reflect.Array;
+import java.lang.reflect.Method;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -27,6 +28,7 @@
 import java.util.ListIterator;
 import java.util.Optional;
 import java.util.Set;
+import java.util.Spliterator;
 import java.util.function.Consumer;
 import java.util.stream.Collector;
 import java.util.stream.DoubleStream;
@@ -35,7 +37,9 @@
 import java.util.stream.Stream;
 
 import org.apiguardian.api.API;
+import org.junit.platform.commons.JUnitException;
 import org.junit.platform.commons.PreconditionViolationException;
+import org.junit.platform.commons.function.Try;
 
 /**
  * Collection of utilities for working with {@link Collection Collections}.
@@ -122,7 +126,7 @@ public static <T> Set<T> toSet(T[] values) {
 	 * returned, so if more control over the returned list is required,
 	 * consider creating a new {@code Collector} implementation like the
 	 * following:
-	 *
+	 * <p>
 	 * <pre class="code">
 	 * public static &lt;T&gt; Collector&lt;T, ?, List&lt;T&gt;&gt; toUnmodifiableList(Supplier&lt;List&lt;T&gt;&gt; listSupplier) {
 	 *     return Collectors.collectingAndThen(Collectors.toCollection(listSupplier), Collections::unmodifiableList);
@@ -161,7 +165,11 @@ public static boolean isConvertibleToStream(Class<?> type) {
 				|| Iterable.class.isAssignableFrom(type)//
 				|| Iterator.class.isAssignableFrom(type)//
 				|| Object[].class.isAssignableFrom(type)//
-				|| (type.isArray() && type.getComponentType().isPrimitive()));
+				|| (type.isArray() && type.getComponentType().isPrimitive())//
+				|| Arrays.stream(type.getMethods())//
+						.filter(m -> m.getName().equals("iterator"))//
+						.map(Method::getReturnType)//
+						.anyMatch(returnType -> returnType == Iterator.class));
 	}
 
 	/**
@@ -177,6 +185,7 @@ public static boolean isConvertibleToStream(Class<?> type) {
 	 * <li>{@link Iterator}</li>
 	 * <li>{@link Object} array</li>
 	 * <li>primitive array</li>
+	 * <li>An object that contains a method with name `iterator` returning an Iterator object</li>
 	 * </ul>
 	 *
 	 * @param object the object to convert into a stream; never {@code null}
@@ -223,8 +232,31 @@ public static Stream<?> toStream(Object object) {
 		if (object.getClass().isArray() && object.getClass().getComponentType().isPrimitive()) {
 			return IntStream.range(0, Array.getLength(object)).mapToObj(i -> Array.get(object, i));
 		}
-		throw new PreconditionViolationException(
-			"Cannot convert instance of " + object.getClass().getName() + " into a Stream: " + object);
+		return tryConvertToStreamByReflection(object);
+	}
+
+	private static Stream<?> tryConvertToStreamByReflection(Object object) {
+		Preconditions.notNull(object, "Object must not be null");
+		try {
+			String name = "iterator";
+			Method method = object.getClass().getMethod(name);
+			if (method.getReturnType() == Iterator.class) {
+				return stream(() -> tryIteratorToSpliterator(object, method), ORDERED, false);
+			}
+			else {
+				throw new PreconditionViolationException(
+					"Method with name 'iterator' does not return " + Iterator.class.getName());
+			}
+		}
+		catch (NoSuchMethodException | IllegalStateException e) {
+			throw new PreconditionViolationException(//
+				"Cannot convert instance of " + object.getClass().getName() + " into a Stream: " + object, e);
+		}
+	}
+
+	private static Spliterator<?> tryIteratorToSpliterator(Object object, Method method) {
+		return Try.call(() -> spliteratorUnknownSize((Iterator<?>) method.invoke(object), ORDERED))//
+				.getOrThrow(e -> new JUnitException("Cannot invoke method " + method.getName() + " onto " + object, e));//
 	}
 
 	/**

jupiter-tests/src/test/kotlin/org/junit/jupiter/api/KotlinDynamicTests.kt

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2015-2025 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+package org.junit.jupiter.api
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.DynamicTest.dynamicTest
+import java.math.BigDecimal
+import java.math.BigDecimal.ONE
+import java.math.MathContext
+import java.math.BigInteger as BigInt
+import java.math.RoundingMode as Rounding
+
+/**
+ * Unit tests for JUnit Jupiter [TestFactory] use in kotlin classes.
+ *
+ * @since 5.12
+ */
+class KotlinDynamicTests {
+    @Nested
+    inner class SequenceReturningTestFactoryTests {
+        @TestFactory
+        fun `Dynamic tests returned as Kotlin sequence`() =
+            generateSequence(0) { it + 2 }
+                .map { dynamicTest("$it should be even") { assertEquals(0, it % 2) } }
+                .take(10)
+
+        @TestFactory
+        fun `Consecutive fibonacci nr ratios, should converge to golden ratio as n increases`(): Sequence<DynamicTest> {
+            val scale = 5
+            val goldenRatio =
+                (ONE + 5.toBigDecimal().sqrt(MathContext(scale + 10, Rounding.HALF_UP)))
+                    .divide(2.toBigDecimal(), scale, Rounding.HALF_UP)
+
+            fun shouldApproximateGoldenRatio(
+                cur: BigDecimal,
+                next: BigDecimal
+            ) = next.divide(cur, scale, Rounding.HALF_UP).let {
+                dynamicTest("$cur / $next = $it should approximate the golden ratio in $scale decimals") {
+                    assertEquals(goldenRatio, it)
+                }
+            }
+            return generateSequence(BigInt.ONE to BigInt.ONE) { (cur, next) -> next to cur + next }
+                .map { (cur) -> cur.toBigDecimal() }
+                .zipWithNext(::shouldApproximateGoldenRatio)
+                .drop(14)
+                .take(10)
+        }
+    }
+}

jupiter-tests/src/test/kotlin/org/junit/jupiter/params/aggregator/KotlinParameterizedTests.kt

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2015-2025 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+package org.junit.jupiter.params.aggregator
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.Arguments.arguments
+import org.junit.jupiter.params.provider.MethodSource
+import java.time.Month
+
+/**
+ * Tests for ParameterizedTest kotlin compatibility
+ */
+object KotlinParameterizedTests {
+    @ParameterizedTest
+    @MethodSource("dataProvidedByKotlinSequence")
+    fun `a method source can be supplied by a Sequence returning method`(
+        value: Int,
+        month: Month
+    ) {
+        assertEquals(value, month.value)
+    }
+
+    @JvmStatic
+    private fun dataProvidedByKotlinSequence() =
+        sequenceOf(
+            arguments(1, Month.JANUARY),
+            arguments(3, Month.MARCH),
+            arguments(8, Month.AUGUST),
+            arguments(5, Month.MAY),
+            arguments(12, Month.DECEMBER)
+        )
+}

platform-tests/src/test/java/org/junit/platform/commons/util/CollectionUtilsTests.java

@@ -25,6 +25,8 @@
 import java.util.Iterator;
 import java.util.List;
 import java.util.Set;
+import java.util.Spliterator;
+import java.util.Spliterators;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.stream.DoubleStream;
 import java.util.stream.IntStream;
@@ -139,6 +141,7 @@ class StreamConversion {
 				Collection.class, //
 				Iterable.class, //
 				Iterator.class, //
+				IteratorProvider.class, //
 				Object[].class, //
 				String[].class, //
 				int[].class, //
@@ -160,10 +163,11 @@ static Stream<Object> objectsConvertibleToStreams() {
 				Stream.of("cat", "dog"), //
 				DoubleStream.of(42.3), //
 				IntStream.of(99), //
-				LongStream.of(100000000), //
+				LongStream.of(100_000_000), //
 				Set.of(1, 2, 3), //
 				Arguments.of((Object) new Object[] { 9, 8, 7 }), //
-				new int[] { 5, 10, 15 }//
+				new int[] { 5, 10, 15 }, //
+				IteratorProvider.of(new Integer[] { 1, 2, 3, 4, 5 })//
 			);
 		}
 
@@ -174,6 +178,8 @@ static Stream<Object> objectsConvertibleToStreams() {
 				Object.class, //
 				Integer.class, //
 				String.class, //
+				IteratorProviderNotUsable.class, //
+				Spliterator.class, //
 				int.class, //
 				boolean.class //
 		})
@@ -242,7 +248,7 @@ void toStreamWithLongStream() {
 		}
 
 		@Test
-		@SuppressWarnings({ "unchecked", "serial" })
+		@SuppressWarnings({ "unchecked" })
 		void toStreamWithCollection() {
 			var collectionStreamClosed = new AtomicBoolean(false);
 			Collection<String> input = new ArrayList<>() {
@@ -287,6 +293,24 @@ void toStreamWithIterator() {
 			assertThat(result).containsExactly("foo", "bar");
 		}
 
+		@Test
+		@SuppressWarnings("unchecked")
+		void toStreamWithIteratorProvider() {
+			final var input = IteratorProvider.of(new String[] { "foo", "bar" });
+
+			final var result = (Stream<String>) CollectionUtils.toStream(input);
+
+			assertThat(result).containsExactly("foo", "bar");
+		}
+
+		@Test
+		void throwWhenIteratorNamedMethodDoesNotReturnAnIterator() {
+			var o = IteratorProviderNotUsable.of(new String[] { "Test" });
+			var e = assertThrows(PreconditionViolationException.class, () -> CollectionUtils.toStream(o));
+
+			assertEquals("Method with name 'iterator' does not return java.util.Iterator", e.getMessage());
+		}
+
 		@Test
 		@SuppressWarnings("unchecked")
 		void toStreamWithArray() {
@@ -355,4 +379,29 @@ public Object convert(Object source, ParameterContext context) throws ArgumentCo
 			}
 		}
 	}
+
+	/**
+	 * An interface that has a method with name 'iterator', returning a java.util/Iterator as a return type
+	 */
+	private interface IteratorProvider<T> {
+
+		@SuppressWarnings("unused")
+		Iterator<T> iterator();
+
+		static <T> IteratorProvider<T> of(T[] elements) {
+			return () -> Spliterators.iterator(Arrays.spliterator(elements));
+		}
+	}
+
+	/**
+	 * An interface that has a method with name 'iterator', but does not return java.util/Iterator as a return type
+	 */
+	private interface IteratorProviderNotUsable {
+		@SuppressWarnings("unused")
+		Object iterator();
+
+		static <T> IteratorProviderNotUsable of(T[] elements) {
+			return () -> Spliterators.iterator(Arrays.spliterator(elements));
+		}
+	}
 }