Add Java 10 Collectors APIs

user/super/com/google/gwt/emul/java/util/stream/Collectors.java

@@ -18,6 +18,7 @@
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.Comparator;
 import java.util.DoubleSummaryStatistics;
 import java.util.HashMap;
@@ -27,6 +28,7 @@
 import java.util.List;
 import java.util.LongSummaryStatistics;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
 import java.util.StringJoiner;
@@ -339,6 +341,11 @@ public static <T> Collector<T,?,List<T>> toList() {
     return toCollection(ArrayList::new);
   }
 
+  public static <T> Collector<T, ?, List<T>> toUnmodifiableList() {
+    Collector<T, ?, List<T>> mapping = mapping(Objects::requireNonNull, toList());
+    return collectingAndThen(mapping, Collections::unmodifiableList);
+  }
+
   public static <T, K, U> Collector<T, ?, Map<K, U>> toMap(
       final Function<? super T, ? extends K> keyMapper,
       final Function<? super T, ? extends U> valueMapper) {
@@ -357,6 +364,29 @@ public static <T> Collector<T,?,List<T>> toList() {
     return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);
   }
 
+  public static <T, K, U> Collector<T, ?, Map<K, U>> toUnmodifiableMap(
+      Function<? super T, ? extends K> keyMapper,
+      Function<? super T, ? extends U> valueMapper) {
+    return collectingAndThen(
+            toMap(disallowNulls(keyMapper), disallowNulls(valueMapper)),
+            Collections::unmodifiableMap
+    );
+  }
+
+  public static <T, K, U> Collector<T, ?, Map<K, U>> toUnmodifiableMap(
+      Function<? super T, ? extends K> keyMapper,
+      Function<? super T, ? extends U> valueMapper,
+      BinaryOperator<U> mergeFunction) {
+    return collectingAndThen(
+            toMap(disallowNulls(keyMapper), disallowNulls(valueMapper), mergeFunction),
+            Collections::unmodifiableMap
+    );
+  }
+
+  private static <T, R> Function<T, R> disallowNulls(Function<T, R> func) {
+    return func.andThen(Objects::requireNonNull);
+  }
+
   public static <T, K, U, M extends Map<K, U>> Collector<T, ?, M> toMap(
       final Function<? super T, ? extends K> keyMapper,
       final Function<? super T, ? extends U> valueMapper,
@@ -389,6 +419,11 @@ public static <T> Collector<T,?,Set<T>> toSet() {
     );
   }
 
+  public static <T> Collector<T, ?, Set<T>> toUnmodifiableSet() {
+    Collector<T, ?, Set<T>> mapping = mapping(Objects::requireNonNull, toSet());
+    return collectingAndThen(mapping, Collections::unmodifiableSet);
+  }
+
   private static <T, D, A> D streamAndCollect(Collector<? super T, A, D> downstream, List<T> list) {
     A a = downstream.supplier().get();
     for (T t : list) {

user/test/com/google/gwt/emultest/EmulJava10Suite.java

@@ -19,13 +19,15 @@
 import com.google.gwt.emultest.java10.util.OptionalIntTest;
 import com.google.gwt.emultest.java10.util.OptionalLongTest;
 import com.google.gwt.emultest.java10.util.OptionalTest;
+import com.google.gwt.emultest.java10.util.stream.CollectorsTest;
 import org.junit.runner.RunWith;
 import org.junit.runners.Suite;
 import org.junit.runners.Suite.SuiteClasses;
 
 /** Test JRE emulations. */
 @RunWith(Suite.class)
 @SuiteClasses({
+        CollectorsTest.class,
         OptionalDoubleTest.class,
         OptionalIntTest.class,
         OptionalLongTest.class,

user/test/com/google/gwt/emultest/java10/util/stream/CollectorsTest.java

@@ -0,0 +1,162 @@
+/*
+ * Copyright 2023 Google Inc.
+ *
+ * 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.gwt.emultest.java10.util.stream;
+
+import static com.google.gwt.emultest.java8.util.stream.CollectorsTest.applyItems;
+import static java.util.stream.Collectors.toUnmodifiableList;
+import static java.util.stream.Collectors.toUnmodifiableMap;
+import static java.util.stream.Collectors.toUnmodifiableSet;
+
+import com.google.gwt.emultest.java.util.EmulTestBase;
+
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+/**
+ * Tests for java.util.stream.Collectors Java 10 API emulation.
+ */
+public class CollectorsTest extends EmulTestBase {
+  private static <T> boolean unmodifiableCollection(Collection<T> c, T existingSample, T newSample) {
+    try {
+      c.remove(existingSample);
+      return false;
+    } catch (UnsupportedOperationException ignore) {
+      // expected
+    }
+    try {
+      c.add(newSample);
+      return false;
+    } catch (UnsupportedOperationException ignore) {
+      // expected
+    }
+    Iterator<T> itr = c.iterator();
+    itr.next();
+    try {
+      itr.remove();
+      return false;
+    } catch (UnsupportedOperationException e) {
+      // expected
+    }
+    return true;
+  }
+
+  public void testToUnmodifiableList() {
+    applyItems(List.of("a", "b"), toUnmodifiableList(), "a", "b", (expected, actual) -> {
+      if (!expected.equals(actual)) {
+        return false;
+      }
+
+      if (!unmodifiableCollection(actual, "a", "z")) {
+        return false;
+      }
+
+      return true;
+    });
+
+    // verify nulls fail
+    try {
+      Stream.of("a").map(ignore -> null).collect(toUnmodifiableList());
+      fail("Expected NPE");
+    } catch (NullPointerException ignore) {
+      // expected
+    }
+  }
+
+  public void testToUnmodifiableMap() {
+    // verify simple cases copy all values and results are unmodifiable
+    applyItems(Map.of("a", 0, "b", 1), toUnmodifiableMap(Function.identity(),
+        k -> k.charAt(0) - 'a'), "a", "b", (expected, actual) -> {
+      if (!expected.equals(actual)) {
+        return false;
+      }
+
+      if (!unmodifiableMap(actual, "a", 0, "z", 100)) {
+        return false;
+      }
+
+      return true;
+    });
+
+    // verify merge works with only one key (but this is just passing through to the toMap func
+    // anyway...)
+    applyItems(Map.of("a", 2), toUnmodifiableMap(Function.identity(), ignore -> 1, Integer::sum),
+        "a", "a");
+
+    // verify nulls blow up for both keys and values
+    try {
+      Stream.of("a").collect(toUnmodifiableMap(obj -> null, Function.identity()));
+      fail("Expected NPE");
+    } catch (NullPointerException ignore) {
+      // expected
+    }
+    try {
+      Stream.of("a").collect(toUnmodifiableMap(Function.identity(), obj -> null));
+      fail("Expected NPE");
+    } catch (Exception ignore) {
+      // expected
+    }
+  }
+
+  private <K, V> boolean unmodifiableMap(Map<K, V> a, K existingKey, V existingValue, K newKey,
+      V newValue) {
+    if (!unmodifiableCollection(a.keySet(), existingKey, newKey)) {
+      return false;
+    }
+    if (!unmodifiableCollection(a.values(), existingValue, newValue)) {
+      return false;
+    }
+
+    try {
+      a.put(newKey, newValue);
+      return false;
+    } catch (Exception ignore) {
+      // expected
+    }
+    try {
+      a.remove(existingKey);
+      return false;
+    } catch (Exception ignore) {
+      // expected
+    }
+
+    return true;
+  }
+
+  public void testToUnmodifiableSet() {
+    applyItems(Set.of("a", "b"), toUnmodifiableSet(), "a", "b", (expected, actual) -> {
+      if (!expected.equals(actual)) {
+        return false;
+      }
+      if (!unmodifiableCollection(actual, "a", "z")) {
+        return false;
+      }
+      return true;
+    });
+
+    // verify nulls fail
+    try {
+      Stream.of("a").map(ignore -> null).collect(toUnmodifiableSet());
+      fail("Expected NPE");
+    } catch (NullPointerException ignore) {
+      // expected
+    }
+  }
+}