Learning Java

Creating Custom Components

In this chapter and the previous chapter, we’ve worked with many different user interface objects and made a lot of new classes that are sort of like components. Our new classes do one particular thing well; a number of them can be added to applets or other containers just like the standard Swing components; and several of them are lightweight components that use system resources efficiently because they don’t rely on a peer. But we haven’t created new components; we’ve just used Swing’s impressive repertoire of components as building blocks. In this section, we’ll create an entirely new component, a dial.

Up until now, our new classes still haven’t really been components. If you think about it, all our classes have been fairly self-contained; they know everything about what to do and don’t rely on other parts of the program to do much processing. Therefore, they are overly specialized. Our menu example created a DinnerFrame class that had a menu of dinner options, but it included all the processing needed to handle the user’s selections. If we wanted to process the selections differently, we’d have to modify the class. A true component separates the detection of user choices from the processing of those choices. It lets the user take some action and then calls another part of the program to process the action.

Generating Events

So we need a way for our new classes to communicate with other parts of the program. Since we want our new classes to be components, ...