8215401: Add isEmpty default method to CharSequence

Reviewed-by: jlaskey, rriggs, jjg, alanb, smarks, darcy

src/java.base/share/classes/java/lang/CharSequence.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 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
@@ -87,6 +87,21 @@ public interface CharSequence {
      */
     char charAt(int index);
 
+    /**
+     * Returns {@code true} if this character sequence is empty.
+     *
+     * @implSpec
+     * The default implementation returns the result of calling {@code length() == 0}.
+     *
+     * @return {@code true} if {@link #length()} is {@code 0}, otherwise
+     * {@code false}
+     *
+     * @since 15
+     */
+    default boolean isEmpty() {
+        return this.length() == 0;
+    }
+
     /**
      * Returns a {@code CharSequence} that is a subsequence of this sequence.
      * The subsequence starts with the {@code char} value at the specified index and

src/java.base/share/classes/java/lang/String.java

@@ -684,6 +684,7 @@ public final class String
      *
      * @since 1.6
      */
+    @Override
     public boolean isEmpty() {
         return value.length == 0;
     }