diff --git a/guava-tests/test/com/google/common/collect/MoreGatherersTest.java b/guava-tests/test/com/google/common/collect/MoreGatherersTest.java
new file mode 100644
index 000000000000..cafccdf47967
--- /dev/null
+++ b/guava-tests/test/com/google/common/collect/MoreGatherersTest.java
@@ -0,0 +1,122 @@
+/*
+ * Copyright (C) 2026 The Guava Authors
+ *
+ * Licensed 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 com.google.common.collect;
+
+import static com.google.common.collect.MoreGatherers.distinctBy;
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.annotations.GwtIncompatible;
+import java.util.Arrays;
+import java.util.List;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Function;
+import java.util.stream.Stream;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
+import org.junit.Test;
+
+/// Tests for `MoreGatherers`.
+@GwtIncompatible
+@NullMarked
+public class MoreGatherersTest {
+
+ @Test
+ public void testDistinctBy_example() {
+ ImmutableList<string> input = ImmutableList.of("foo", "bar", "quux", "womble");
+ List<string> output = input.stream().gather(distinctBy(String::length)).toList();
+ assertThat(output).containsExactly("foo", "quux", "womble").inOrder();
+ }
+
+ @Test
+ public void testDistinctBy_empty() {
+ List<string> output = Stream.<string>empty().gather(distinctBy(String::length)).toList();
+ assertThat(output).isEmpty();
+ }
+
+ @Test
+ public void testDistinctBy_allDistinct() {
+ ImmutableList<string> input = ImmutableList.of("a", "bb", "ccc", "dddd");
+ List<string> output = input.stream().gather(distinctBy(String::length)).toList();
+ assertThat(output).containsExactly("a", "bb", "ccc", "dddd").inOrder();
+ }
+
+ @Test
+ public void testDistinctBy_allSame() {
+ ImmutableList<string> input = ImmutableList.of("a", "b", "c", "d");
+ List<string> output = input.stream().gather(distinctBy(String::length)).toList();
+ assertThat(output).containsExactly("a");
+ }
+
+ @Test
+ public void testDistinctBy_identity() {
+ ImmutableList<string> input = ImmutableList.of("a", "b", "a", "c", "b");
+ List<string> output = input.stream().gather(distinctBy(Function.identity())).toList();
+ assertThat(output).containsExactly("a", "b", "c").inOrder();
+ }
+
+ @Test
+ public void testDistinctBy_nullKeys() {
+ ImmutableList<string> input = ImmutableList.of("a", "b", "c");
+ List<string> output = input.stream().gather(distinctBy(s -> null)).toList();
+ assertThat(output).containsExactly("a");
+ }
+
+ @Test
+ public void testDistinctBy_customObjects() {
+ record Item(int key, String value) {}
+ ImmutableList<item> input =
+ ImmutableList.of(
+ new Item(1, "one-a"),
+ new Item(2, "two-a"),
+ new Item(1, "one-b"),
+ new Item(3, "three-a"),
+ new Item(2, "two-b"));
+ List<item> output = input.stream().gather(distinctBy(Item::key)).toList();
+ assertThat(output)
+ .containsExactly(new Item(1, "one-a"), new Item(2, "two-a"), new Item(3, "three-a"))
+ .inOrder();
+ }
+
+ @Test
+ public void testDistinctBy_shortCircuit() {
+ AtomicInteger seenCount = new AtomicInteger();
+ ImmutableList<string> input = ImmutableList.of("a", "bb", "ccc", "dddd", "eeeee");
+ List<string> output =
+ input.stream()
+ .peek(e -> seenCount.incrementAndGet())
+ .gather(distinctBy(String::length))
+ .limit(2)
+ .toList();
+ assertThat(output).containsExactly("a", "bb").inOrder();
+ assertThat(seenCount.get()).isEqualTo(2);
+ }
+
+ @Test
+ public void testDistinctBy_superTypeFunction() {
+ ImmutableList<string> input = ImmutableList.of("foo", "bar", "quux", "womble");
+ List<string> output = input.stream().gather(distinctBy(CharSequence::length)).toList();
+ assertThat(output).containsExactly("foo", "quux", "womble").inOrder();
+ }
+
+ @Test
+ public void testDistinctBy_nullElements() {
+ List<@nullable String> input = Arrays.asList("foo", null, "bar", null, "quux");
+ List<@nullable String> output =
+ input.stream().gather(distinctBy(s -> s == null ? 0 : s.length())).toList();
+ assertThat(output).containsExactly("foo", null, "quux").inOrder();
+ }
+}
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