The Undo Framework - The Definitive Guide to Java Swing - page 790

And that's it! The last two classes make the UndoDrawing example class (Listing 21-4) work.
When creating your own undoable classes, you will need to subclass a nonundoable class and
then add the necessary support to make it undoable. In addition, you will need to define an
UndoableEdit implementation to support your specific class.
Using an Outside Object to Manage Undo States
In the previous example, it was the responsibility of your custom UndoableEdit implementation
to maintain the before-and-after state of the undoable object. The Swing Undo Framework
also supports the ability of an object outside the undoable edit implementation to manage the
state. When using an outside object for state management, it isn't necessary to implement the
UndoableEdit interface yourself. Instead, you can use the StateEdit class as the UndoableEdit
implementation. The StateEdit class then relies on a class to implement the StateEditable
interface to manage the before-and-after storage of the state of an undoable object (within a
Hashtable ).
StateEditable Interface
The StateEditable interface consists of two methods and a meaningless string constant.
public interface StateEditable {
public final static String RCSID;
public void restoreState(Hashtable state);
public void storeState(Hashtable state);
}
An object that supports the undoing of its operations stores its state with the storeState( ➥
Hashtable) method. This is all the information about the state of the object that can change.
Then restoring the state of the object is done in the restoreState(Hashtable) method.
To demonstrate, let's see how to rewrite the UndoableDrawingPanel example. Using this
interface with the updated, undoable drawing panel involves implementing the interface, and
storing and getting the polygon shown earlier in Figure 21-5. That's because the polygon is the
only state information we care about undoing. The source code is shown in Listing 21-7.
Listing 21-7. The StateEditable Implementation of the Updated Component
public class UndoableDrawingPanel2 extends JPanel implements StateEditable {
private static String POLYGON_KEY = "Polygon";
public void storeState(Hashtable state) {
state.put(POLYGON_KEY, getPolygon());
}