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 int length() {
      *
      * @since 1.6
      */
+    @Override
     public boolean isEmpty() {
         return value.length == 0;
     }

src/java.base/share/classes/java/nio/X-Buffer.java.template

@@ -1645,6 +1645,18 @@ public abstract class $Type$Buffer
         return remaining();
     }
 
+    /**
+     * Returns {@code true} if this character buffer is empty.
+     *
+     * @return {@code true} if there are {@code 0} remaining characters,
+     *         otherwise {@code false}
+     *
+     * @since 15
+     */
+    public final boolean isEmpty() {
+        return remaining() == 0;
+    }
+
     /**
      * Reads the character at the given index relative to the current
      * position.

test/jdk/java/lang/CharSequence/Emptiness.java

@@ -0,0 +1,104 @@
+/*
+ * Copyright (c) 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import java.nio.ByteBuffer;
+import java.nio.CharBuffer;
+
+import org.testng.Assert;
+import org.testng.annotations.Test;
+
+/**
+ * @test
+ * @bug 8215401
+ * @summary Test to verify the isEmpty method is behaviorally consistent
+ *          with length
+ * @run testng Emptiness
+ */
+public class Emptiness {
+
+    @Test
+    public void isEmpty() {
+        checkEmpty(new StringBuilder());
+        checkEmpty(new StringBuffer());
+        checkEmpty("");
+        checkEmpty(new CharSequence() {
+            @Override public int length() { return 0; }
+            @Override public char charAt(int index) { return 'f'; }
+            @Override public CharSequence subSequence(int start, int end) {
+                throw new UnsupportedOperationException();
+            }
+        });
+        checkEmpty(CharBuffer.wrap(new char[0]));
+        checkEmpty(CharBuffer.wrap(""));
+
+        // A CharBuffer being filled is empty when there's no room remaining
+        // - or there's nothing in the buffer after a flip
+        checkEmpty(ByteBuffer.allocate(0).asCharBuffer());
+        checkEmpty(ByteBuffer.allocate(2).asCharBuffer().append('f'));
+        checkEmpty(ByteBuffer.allocate(2).asCharBuffer().flip());
+
+        checkEmpty(ByteBuffer.allocateDirect(0).asCharBuffer());
+        checkEmpty(ByteBuffer.allocateDirect(2).asCharBuffer().append('f'));
+        checkEmpty(ByteBuffer.allocateDirect(2).asCharBuffer().flip());
+    }
+
+    @Test
+    public void isNotEmpty() {
+        checkNotEmpty(new StringBuilder().append("foo"));
+        checkNotEmpty(new StringBuffer().append("bar"));
+        checkNotEmpty("baz");
+        checkNotEmpty(new CharSequence() {
+            @Override public int length() { return 1; }
+            @Override public char charAt(int index) { return 'f'; }
+            @Override public CharSequence subSequence(int start, int end) {
+                throw new UnsupportedOperationException();
+            }
+        });
+        checkNotEmpty(CharBuffer.wrap(new char[] { 'f' }));
+        checkNotEmpty(CharBuffer.wrap("foo"));
+
+        // A CharBuffer being filled is non-empty when there's room remaining
+        // - or when there's something in the buffer after a flip
+        checkNotEmpty(ByteBuffer.allocate(2).asCharBuffer());
+        checkNotEmpty(ByteBuffer.allocate(4).asCharBuffer().append('f'));
+        checkNotEmpty(ByteBuffer.allocate(2).asCharBuffer().append('f').flip());
+
+        checkNotEmpty(ByteBuffer.allocateDirect(2).asCharBuffer());
+        checkNotEmpty(ByteBuffer.allocateDirect(4).asCharBuffer().append('f'));
+        checkNotEmpty(ByteBuffer.allocateDirect(2).asCharBuffer().append('f').flip());
+    }
+
+    public void checkEmpty(CharSequence cs) {
+        Assert.assertTrue(cs.isEmpty());
+        Assert.assertTrue(consistentWithLength(cs));
+    }
+
+    public void checkNotEmpty(CharSequence cs) {
+        Assert.assertTrue(!cs.isEmpty());
+        Assert.assertTrue(consistentWithLength(cs));
+    }
+
+    public boolean consistentWithLength(CharSequence cs) {
+        return cs.isEmpty() == (cs.length() == 0);
+    }
+}