8238684: Override getOrDefault in immutable Map implementation

Reviewed-by: forax, psandoz, smarks

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

@@ -904,6 +904,20 @@ class ImmutableCollections {
         @Override public V replace(K key, V value) { throw uoe(); }
         @Override public boolean replace(K key, V oldValue, V newValue) { throw uoe(); }
         @Override public void replaceAll(BiFunction<? super K,? super V,? extends V> f) { throw uoe(); }
+
+        /**
+         * @implNote {@code null} values are disallowed in these immutable maps,
+         * so we can improve upon the default implementation since a
+         * {@code null} return from {@code get(key)} always means the default
+         * value should be returned.
+         */
+        @Override
+        public V getOrDefault(Object key, V defaultValue) {
+            V v;
+            return ((v = get(key)) != null)
+                    ? v
+                    : defaultValue;
+        }
     }
 
     static final class Map1<K,V> extends AbstractImmutableMap<K,V> {

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

@@ -34,6 +34,9 @@ import java.util.concurrent.TimeUnit;
  */
 @State(Scope.Benchmark)
 @OutputTimeUnit(TimeUnit.MICROSECONDS)
+@Fork(value = 3)
+@Warmup(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
+@Measurement(iterations = 5, time = 2, timeUnit = TimeUnit.SECONDS)
 public class ImmutableColls {
 
     public static String[] STRINGS = {"hi", "all", "of", "you"};
@@ -217,6 +220,13 @@ public class ImmutableColls {
                 fm4.containsValue("hi");
     }
 
+    @Benchmark
+    @CompilerControl(CompilerControl.Mode.DONT_INLINE)
+    public void getOrDefault(Blackhole bh) {
+        bh.consume(fm4.getOrDefault("hi", "test"));
+        bh.consume(fm4.getOrDefault("not_in_this_map", "test"));
+    }
+
     public int sizeOf(List<String> list) {
         return list.size();
     }