Add emulation for Collection.toArray(IntFunction) (#10180)

Fixes #10158

Author: niloc132

user/super/com/google/gwt/emul/java/util/Collection.java

@@ -17,6 +17,7 @@
 
 import static javaemul.internal.InternalPreconditions.checkNotNull;
 
+import java.util.function.IntFunction;
 import java.util.function.Predicate;
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
@@ -87,4 +88,12 @@ default Stream<E> stream() {
 
   @JsIgnore
   <T> T[] toArray(T[] a);
+
+  @JsIgnore
+  default <T> T[] toArray(IntFunction<T[]> factory) {
+    // This differs from the JDK's default implementation, which according to Javadoc
+    // will call the function with zero, as this then requires GWT to clone the array
+    // again before using it.
+    return toArray(factory.apply(size()));
+  }
 }

user/test/com/google/gwt/emultest/EmulJava11Suite.java

@@ -16,6 +16,7 @@
 package com.google.gwt.emultest;
 
 import com.google.gwt.emultest.java11.lang.StringTest;
+import com.google.gwt.emultest.java11.util.CollectionTest;
 import com.google.gwt.emultest.java11.util.OptionalDoubleTest;
 import com.google.gwt.emultest.java11.util.OptionalIntTest;
 import com.google.gwt.emultest.java11.util.OptionalLongTest;
@@ -28,6 +29,7 @@
 /** Test JRE emulations. */
 @RunWith(Suite.class)
 @Suite.SuiteClasses({
+        CollectionTest.class,
         OptionalDoubleTest.class,
         OptionalIntTest.class,
         OptionalLongTest.class,

user/test/com/google/gwt/emultest/java11/util/CollectionTest.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2025 GWT Project 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.gwt.emultest.java11.util;
+
+import com.google.gwt.emultest.java.util.EmulTestBase;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Set;
+
+/**
+ * Tests for java.util.Collection Java 11 API emulation.
+ * <p>
+ * We can't easily test Collection methods themselves, so trying a few basic implementations.
+ */
+public class CollectionTest extends EmulTestBase {
+
+  public void testListArrayOfMethodRef() {
+    String[] arr = List.of("a", "b").toArray(String[]::new);
+    assertEquals(2, arr.length);
+    assertTrue(arr instanceof String[]);
+
+    assertEquals("a", arr[0]);
+    assertEquals("b", arr[1]);
+  }
+
+  public void testSetArrayOfMethodRef() {
+    Set<String> ab = Set.of("a", "b");
+    String[] arr = ab.toArray(String[]::new);
+    assertEquals(2, arr.length);
+    assertTrue(arr instanceof String[]);
+    assertTrue(ab.containsAll(Arrays.asList(arr)));
+  }
+}