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Sub-classing AWT classesA simple way to allow additional functionality to be added to basic AWT classes. There are limitations to the user interface elements that can be created this way. The easiest way to create a new user interface element is to define a subclass of an AWT class that is similar. For example, you could create a class called RedButton which was identical to the standard Button class except for having a red background (the default is grey). The code for this might look like:
But we could not use this method to create, say, a round button, or a button that pops up a message when the mouse moves over it. Why not? Class Button is an abstraction of the 'real' button that appears on the display. It only offers functionality that is common to all or most real computer systems. Few real graphical user interfaces allow circular buttons to be created automatically, so class Button cannot provide this. This is a necessary price to pay for portability, but it can be very frustrating. So how could you create a circular button? Probably the easiest way is to create a subclass of a different AWT element. A good candidate here would be Canvas. A Canvas object is simply a blank, rectangular region of the screen. Your program would have to draw the button as it would appear, and take appropriate action when the user clicks on it. This is significantly more complex than subclassing Button. Look back to the Button1.java applet - perhaps try changing it to use the RedButton class above.
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