8358526: Clarify behavior of java.awt.HeadlessException constructed with no-args

src/java.desktop/share/classes/java/awt/HeadlessException.java

@@ -48,7 +48,9 @@ public class HeadlessException extends UnsupportedOperationException {
     private static final long serialVersionUID = 167183644944358563L;
 
     /**
-     * Constructs new {@code HeadlessException} with empty message.
+     * Constructs a new {@code HeadlessException} with {@code null} as its detail message.
+     * The default headless message may replace {@code null} in some cases, as outlined below.
+     * <p>
      * For such {@code HeadlessException} the default headless error message
      * may be auto-generated for some platforms.
      * The text of the default headless message may depend on

test/jdk/java/awt/Headless/HeadlessExceptionTest.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2025, 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.awt.HeadlessException;
+
+/*
+ * @test 8358526
+ * @summary Verify behaviour of no-args HeadlessException and getMessage
+ * @run main/othervm -Djava.awt.headless=true HeadlessExceptionTest
+ * @run main/othervm HeadlessExceptionTest
+ */
+
+public class HeadlessExceptionTest {
+
+    public static void main (String[] args) {
+        String nullmsg = new HeadlessException().getMessage();
+        String emptymsg = new HeadlessException("").getMessage();
+        System.out.println("nullmsg=" + nullmsg);
+        System.out.println("emptymsg=" + emptymsg);
+        if (nullmsg != null) {
+            if ("".equals(nullmsg)) {
+                throw new RuntimeException("empty message instead of null");
+            }
+            if (!nullmsg.equals(emptymsg)) {
+                throw new RuntimeException("non-null messages differ");
+            }
+        }
+    }
+}