Summary

Update Swing classes to adopt the new sealed modifier, making sub-classes non-sealed of final as appropriate

Problem

There are a number of Swing classes which do not permit direct application sub-classing since they do not provide public or protected constructors, but it is not as clear as it could be if they permit just some sub-classes or are effectively final.

Solution

Update to use the new JEP 409 sealed and non-sealed modifiers to make the design intent clearer.

Specification

Update the following nested classes of javax.swing.GroupLayout as below

-     public abstract class Group extends Spring
+     public abstract sealed class Group extends Spring
+         permits ParallelGroup,
+                 SequentialGroup

-     public class SequentialGroup extends Group
+     public final class SequentialGroup extends Group

-     public class ParallelGroup extends Group
+     public sealed class ParallelGroup extends Group
+          permits BaselineGroup

-     private class BaselineGroup extends ParallelGroup
+     private final class BaselineGroup extends ParallelGroup

Update javax.swing.ToolTipManager

- public class ToolTipManager extends MouseAdapter implements MouseMotionListener  
+ public final class ToolTipManager extends MouseAdapter implements MouseMotionListener  

Update the following nested classes of javax.swing.plaf.basic.BasicSplitPaneUI

-     public class BasicHorizontalLayoutManager implements LayoutManager2
+     public sealed class BasicHorizontalLayoutManager implements LayoutManager2
+          permits BasicVerticalLayoutManager

-     public class BasicVerticalLayoutManager extends
+     public non-sealed class BasicVerticalLayoutManager extends

Update javax.swing.text.StyleConstants and its following nested classes

- public class StyleConstants 
+ public sealed class StyleConstants
+     permits StyleConstants.CharacterConstants,
+             StyleConstants.ColorConstants,
+             StyleConstants.FontConstants,
+             StyleConstants.ParagraphConstants 

-     public static class ParagraphConstants extends StyleConstants
+     public static final class ParagraphConstants extends StyleConstants

-     public static class CharacterConstants extends StyleConstants
+     public static final class CharacterConstants extends StyleConstants


-     public static class ColorConstants extends StyleConstants
+     public static final class ColorConstants extends StyleConstants

-     public static class FontConstants extends StyleConstants
+     public static final class FontConstants extends StyleConstants

Update javax.swing.text.html.FormSubmitEvent to be final

- public class FormSubmitEvent extends HTMLFrameHyperlinkEvent 
+ public final class FormSubmitEvent extends HTMLFrameHyperlinkEvent 

Update nested classes of javax.swing.text.html.StyleSheet to be final

    @SuppressWarnings("serial") // Same-version serialization only
-     public static class BoxPainter implements Serializable 
+     public static final class BoxPainter implements Serializable 

    @SuppressWarnings("serial") // Same-version serialization only
-     public static class ListPainter implements Serializable 
+     public static final class ListPainter implements Serializable