6463708: DefaultButtonModel.setMnemonic generates ChangeEvent for no change

Reviewed-by: azvegint, prr

src/java.desktop/share/classes/javax/swing/DefaultButtonModel.java

@@ -290,8 +290,10 @@ public class DefaultButtonModel implements ButtonModel, Serializable {
      * {@inheritDoc}
      */
     public void setMnemonic(int key) {
-        mnemonic = key;
-        fireStateChanged();
+        if (this.mnemonic != key) {
+            this.mnemonic = key;
+            fireStateChanged();
+        }
     }
 
     /**

test/jdk/javax/swing/DefaultButtonModel/TestMnemonicEvent.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2022, 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.
+ */
+
+/*
+ * @test
+ * @bug 6463708
+ * @summary Verifies if DefaultButtonModel.setMnemonic
+ *          generates ChangeEvent for no change
+ * @run main TestMnemonicEvent
+ */
+
+import javax.swing.DefaultButtonModel;
+import javax.swing.event.ChangeEvent;
+import javax.swing.event.ChangeListener;
+
+public class TestMnemonicEvent implements ChangeListener {
+
+    private static ChangeEvent lastEvent;
+
+    public void stateChanged(ChangeEvent e) {
+        lastEvent = e;
+    }
+
+    public static void main(String[] args) {
+        TestMnemonicEvent test = new TestMnemonicEvent();
+        DefaultButtonModel m = new DefaultButtonModel();
+        m.addChangeListener(test);
+        m.setMnemonic(70);
+        System.out.println("Event triggered: " + (lastEvent != null));
+
+        // clear the recorded event, set the same mnemonic then check if another
+        // event is triggered...
+        lastEvent = null;
+        m.setMnemonic(70);
+        System.out.println("Event triggered: " + (lastEvent != null));
+        if (lastEvent != null) {
+            throw new RuntimeException("ChangeEvent triggered for same mnemonic");
+        }
+    }
+}