For listener interfaces where it's common to just implement one or two methods, there are corresponding Adapter classes with empty method implementations. (For example: MouseListener/MouseAdapter, KeyListener/KeyAdapter, ComponentListener/ComponentAdapter, ...)
The Adapter classes are really convenient for obvious reasons. However, due to the constraint of single inheritance, they can't be used together with each other. That is, if I want to listen to key presses (KeyListener.keyPressed) and mouse clicks (MouseListener.mouseClicked) I'm doomed to either 1) choose to extend one of the adapters and clutter my code with empty method implementations for the other, or 2) Extend the two adapters in two separate classes and instantiate two separate listener objects (which gets slightly messy if they need to share some state). Therefor I propose to add empty default implementations for these types of listener interfaces. This would allow me to for instance implement KeyListener and MouseListener and just override keyPressed and mouseClicked. Some basic grepping yields the following list: ComponentListener ContainerListener DragSourceListener DropTargetListener FocusListener HierarchyBoundsListener InternalFrameListener KeyListener MouseListener MouseMotionListener PrintJobListener WindowListener best regards, Andreas Lundblad
