Make Lists.transform() views delegate spliterator()/forEach() to the backing list #8589

Original file line number	Diff line number	Diff line change
Expand Up		@@ -61,6 +61,7 @@
		import java.util.ListIterator;
		import java.util.NoSuchElementException;
		import java.util.RandomAccess;
	import java.util.Spliterator;
		import java.util.concurrent.CopyOnWriteArrayList;
		import junit.framework.Test;
		import junit.framework.TestCase;
Expand Down Expand Up		@@ -822,6 +823,62 @@ public void testTransformIteratorSequential() {
		assertTransformIterator(list);
		}

	@GwtIncompatible // CopyOnWriteArrayList, Spliterator
	public void testTransformSpliteratorDoesNotThrowConcurrentModificationException() {
	// CopyOnWriteArrayList is documented to never throw ConcurrentModificationException; Lists
	// .transform() is documented to return a threadsafe list when the input list and function are
	// threadsafe. The spliterator of the transformed list must honor that by delegating to the
	// backing list's own (snapshotting) spliterator, rather than falling back to the JDK's default
	// index-based RandomAccessSpliterator, which translates concurrent structural changes into a
	// ConcurrentModificationException.
	CopyOnWriteArrayList<Integer> fromList = new CopyOnWriteArrayList<>(SOME_LIST);
	List<String> transformed = transform(fromList, SOME_FUNCTION);

	Spliterator<String> spliterator = transformed.spliterator();
	List<String> results = new ArrayList<>();
	assertTrue(spliterator.tryAdvance(results::add));

	// Structurally mutate the backing list after the spliterator was created but before it has
	// finished being consumed.
	fromList.clear();

	// Must not throw, and must still report the elements from the original snapshot, exactly as
	// fromList.spliterator() itself would.
	spliterator.forEachRemaining(results::add);
	assertEquals(SOME_STRING_LIST, results);
	}

	@GwtIncompatible // CopyOnWriteArrayList, Spliterator
	public void testTransformSpliteratorPropagatesBackingListCharacteristics() {
	CopyOnWriteArrayList<Integer> fromList = new CopyOnWriteArrayList<>(SOME_LIST);
	List<String> transformed = transform(fromList, SOME_FUNCTION);

	// CopyOnWriteArrayList.spliterator() reports IMMUTABLE (among other bits); a spliterator that
	// instead falls back to the default RandomAccessSpliterator would not, since that
	// implementation has no way of knowing the backing list is safe to snapshot.
	assertTrue(transformed.spliterator().hasCharacteristics(Spliterator.IMMUTABLE));
	}

	@GwtIncompatible // Spliterator
	public void testTransformSpliteratorRandomAccess() {
	List<Integer> fromList = new ArrayList<>(SOME_LIST);
	List<String> transformed = transform(fromList, SOME_FUNCTION);

	List<String> results = new ArrayList<>();
	transformed.spliterator().forEachRemaining(results::add);
	assertEquals(SOME_STRING_LIST, results);
	}

	@GwtIncompatible // Spliterator
	public void testTransformForEachRandomAccess() {
	List<Integer> fromList = new ArrayList<>(SOME_LIST);
	List<String> transformed = transform(fromList, SOME_FUNCTION);

	List<String> results = new ArrayList<>();
	transformed.forEach(results::add);
	assertEquals(SOME_STRING_LIST, results);
	}

		/**
		* This test depends on the fact that {@code AbstractSequentialList.iterator} transforms the
		* {@code iterator()} call into a call on {@code listIterator(int)}. This is fine because the
Expand Down

14 changes: 14 additions & 0 deletions guava/src/com/google/common/collect/Lists.java

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -51,7 +51,9 @@
		import java.util.NoSuchElementException;
		import java.util.Objects;
		import java.util.RandomAccess;
	import java.util.Spliterator;
		import java.util.concurrent.CopyOnWriteArrayList;
	import java.util.function.Consumer;
		import java.util.function.Predicate;
		import org.jspecify.annotations.Nullable;

Expand Down Expand Up		@@ -661,6 +663,18 @@ public boolean isEmpty() {
		return fromList.isEmpty();
		}

	@Override
	@GwtIncompatible // Spliterator
	public Spliterator<T> spliterator() {
	return CollectSpliterators.map(fromList.spliterator(), 0, function);
	}

	@Override
	public void forEach(Consumer<? super T> action) {
	checkNotNull(action);
	fromList.forEach((F f) -> action.accept(function.apply(f)));
	}

		@Override
		public boolean removeIf(Predicate<? super T> filter) {
		checkNotNull(filter);
Expand Down

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