8274715: Implement forEach in Collections.CopiesList

Reviewed-by: martin

src/java.base/share/classes/java/util/Collections.java

@@ -5182,6 +5182,16 @@ public E get(int index) {
             return element;
         }
 
+        @Override
+        public void forEach(Consumer<? super E> action) {
+            Objects.requireNonNull(action);
+            int n = this.n;
+            E element = this.element;
+            for (int i = 0; i < n; i++) {
+                action.accept(element);
+            }
+        }
+
         public Object[] toArray() {
             final Object[] a = new Object[n];
             if (element != null)

test/micro/org/openjdk/bench/java/util/NCopiesBenchmarks.java

@@ -0,0 +1,34 @@
+package micro.org.openjdk.bench.java.util;
+
+import org.openjdk.jmh.annotations.*;
+import org.openjdk.jmh.infra.Blackhole;
+
+import java.util.*;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * @see <a href="https://bugs.openjdk.java.net/browse/JDK-8274715">JDK-8274715</a>
+ */
+@Fork(value = 3)
+@State(Scope.Thread)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@Warmup(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
+@Measurement(iterations = 5, time = 2, timeUnit = TimeUnit.SECONDS)
+public class NCopiesBenchmarks {
+    @Param({"10", "50", "100"})
+    int size;
+
+    private List<Object> list;
+
+    @Setup
+    public void prepare() {
+        list = Collections.nCopies(size, new Object());
+    }
+
+    @Benchmark
+    public void forEach(Blackhole bh) {
+        list.forEach(bh::consume);
+    }
+
+}