Three ways to implement event listeners

Chapter 14 discusses the event listener model. ReboundGUI.java is a graphical program that uses a listener to do a simple animation. In this example, the event is a timer going off, and the listener redraws an image at a new location to implement the animation. In addition to the timer, there is another event and listener: the application listens for mouse clicks and stops the animation or restarts it whenever the mouse is clicked. The angle of travel of the bouncing ball is also changed at each click. The implementation of listeners in this program uses inner classes. But this is not the only way to implement listeners.

There are other ways that event listeners can be built besides using an inner class. In this problem you will change the mouse listener implementation in the application described above to two other ways to implement an event listener. This requires you to consider the relationship of objects of different class types. The other ways that you will implement the listeners are:

• using an external event listener class
• use the GUI class itself as the event listener

Note that you only have to change the implementation of the mouse listener. The timer listener stays the same for all versions.

Steps for this assignment:

1. Download ReboundGUI.java and happyface.gif (right click in your browser on these links to download). Compile ReboundGUI.java and make sure it runs. You should see a happy face bouncing around the screen. If you see a black screen, you may have forgotten to download the gif file. Click to see what happens with each mouse click. This implementation is similar to the examples in the book, using an inner class for ReboundMouseListener. According to the access rules in Java, the inner class has access to the ReboundGUI class and its toggle method to stop/restart the animation.

2. Now make a copy of ReboundGUI.java called ReboundGUI2.java and change the class name to ReboundGUI2. Modify ReboundGUI2.java to use an external class called ReboundMouseListener. That is, remove the inner class and instead implement a similar external class in a separate Java file named ReboundMouseListener.java. Fix up code in ReboundGUI2 so that it all works - there may be several things you have to change. Again you should see a bouncing happy face and the mouse should stop/start the animation.

   Don't forget that you will replace   new ReboundGUI()   with   new ReboundGUI2()   in the original main method as well as any other place that the class name ReboundGUI appears.

   Make sure that you implement the listener in a separate Java file named ReboundMouseListener.java, and remove the inner class ReboundMouseListener definition from ReboundGUI2.java.

3. Now make another copy of the original ReboundGUI.java, but this time call it ReboundGUI3.java, and change the class name to ReboundGUI3. Modify ReboundGUI3.java so that it does not need the separate class ReboundMouseListener at all. To do this, change the ReboundGUI3 class to implement the MouseListener interface itself. Fix up code in ReboundGUI3 so that it all works.

   As above you will replace   new ReboundGUI()   with   new ReboundGUI3()   in the original main method as well as any other place that the class name ReboundGUI appears.

   Make sure that you remove the inner class ReboundMouseListener definition from ReboundGUI3.java, and that there is no class named ReboundMouseListener.

Things to think about

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