8228615: Optional.empty doc should suggest using isEmpty

Reviewed-by: lancea, bpb, naoto

Author: Stuart Marks

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -74,9 +74,9 @@ public final class Optional<T> {
      *
      * @apiNote
      * Though it may be tempting to do so, avoid testing if an object is empty
-     * by comparing with {@code ==} against instances returned by
+     * by comparing with {@code ==} or {@code !=} against instances returned by
      * {@code Optional.empty()}.  There is no guarantee that it is a singleton.
-     * Instead, use {@link #isPresent()}.
+     * Instead, use {@link #isEmpty()} or {@link #isPresent()}.
      *
      * @param <T> The type of the non-existent value
      * @return an empty {@code Optional}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -83,9 +83,9 @@ private OptionalDouble() {
      *
      * @apiNote
      * Though it may be tempting to do so, avoid testing if an object is empty
-     * by comparing with {@code ==} against instances returned by
+     * by comparing with {@code ==} or {@code !=} against instances returned by
      * {@code OptionalDouble.empty()}.  There is no guarantee that it is a singleton.
-     * Instead, use {@link #isPresent()}.
+     * Instead, use {@link #isEmpty()} or {@link #isPresent()}.
      *
      *  @return an empty {@code OptionalDouble}.
      */

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -83,9 +83,9 @@ private OptionalInt() {
      *
      * @apiNote
      * Though it may be tempting to do so, avoid testing if an object is empty
-     * by comparing with {@code ==} against instances returned by
+     * by comparing with {@code ==} or {@code !=} against instances returned by
      * {@code OptionalInt.empty()}.  There is no guarantee that it is a singleton.
-     * Instead, use {@link #isPresent()}.
+     * Instead, use {@link #isEmpty()} or {@link #isPresent()}.
      *
      * @return an empty {@code OptionalInt}
      */

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -83,9 +83,9 @@ private OptionalLong() {
      *
      * @apiNote
      * Though it may be tempting to do so, avoid testing if an object is empty
-     * by comparing with {@code ==} against instances returned by
+     * by comparing with {@code ==} or {@code !=} against instances returned by
      * {@code OptionalLong.empty()}.  There is no guarantee that it is a singleton.
-     * Instead, use {@link #isPresent()}.
+     * Instead, use {@link #isEmpty()} or {@link #isPresent()}.
      *
      * @return an empty {@code OptionalLong}.
      */