Merge pull request #32074: Fixes GitHub issue #30257

sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/SerializableComparator.java

@@ -19,10 +19,29 @@
 
 import java.io.Serializable;
 import java.util.Comparator;
+import java.util.Objects;
+import java.util.function.Function;
 
 /**
  * A {@code Comparator} that is also {@code Serializable}.
  *
  * @param <T> type of values being compared
  */
-public interface SerializableComparator<T> extends Comparator<T>, Serializable {}
+public interface SerializableComparator<T> extends Comparator<T>, Serializable {
+  /**
+   * Analogous to {@link Comparator#comparing(Function)}, except that it takes in a {@link
+   * SerializableFunction} as the key extractor and returns a {@link SerializableComparator}.
+   *
+   * @param keyExtractor the function used to extract the {@link java.lang.Comparable} sort key
+   * @return A {@link SerializableComparator} that compares by an extracted key
+   * @param <T> the type of element to be compared
+   * @param <V> the type of the {@code Comparable} sort key
+   * @see Comparator#comparing(Function)
+   */
+  static <T, V extends Comparable<? super V>> SerializableComparator<T> comparing(
+      SerializableFunction<? super T, ? extends V> keyExtractor) {
+    Objects.requireNonNull(keyExtractor);
+    return (SerializableComparator<T>)
+        (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
+  }
+}

sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/SerializableComparatorTest.java

@@ -0,0 +1,63 @@
+/*
+ * 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.beam.sdk.transforms;
+
+import java.io.Serializable;
+import java.util.function.Function;
+import org.apache.beam.sdk.util.SerializableUtils;
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Tests for {@link SerializableComparator}. */
+@RunWith(JUnit4.class)
+public class SerializableComparatorTest {
+
+  /**
+   * Tests if the {@link SerializableComparator} returned by {@link
+   * SerializableComparator#comparing(SerializableFunction)} using {@link
+   * SerializableUtils#ensureSerializable(Serializable)}.
+   */
+  @Test
+  public void testSerializable() {
+    SerializableFunction<String, Integer> fn = Integer::parseInt;
+
+    SerializableComparator<String> cmp = SerializableComparator.comparing(fn);
+    SerializableUtils.ensureSerializable(cmp);
+  }
+
+  /**
+   * Tests if {@link SerializableComparator#comparing(Function)} throws a {@link
+   * java.lang.NullPointerException} if <code>null</code> is passed to it.
+   */
+  @Test(expected = NullPointerException.class)
+  public void testIfNPEThrownForNullFunction() {
+    SerializableComparator.comparing(null);
+  }
+
+  /** Tests the basic comparison function of the {@link SerializableComparator} returned. */
+  @Test
+  public void testBasicComparison() {
+    SerializableFunction<String, Integer> fn = Integer::parseInt;
+    SerializableComparator<String> cmp = SerializableComparator.comparing(fn);
+
+    Assert.assertTrue(cmp.compare("1", "10") < 0);
+    Assert.assertTrue(cmp.compare("9", "6") > 0);
+  }
+}