Java™ Application Development on Linux - Dator

356Chapter 16Getting in the Swing of Things: Designing a GUI for BudgetProNote that the constructor takes a String argument—that’s the text thatwill appear in the button. A button may also have an icon (image) in it (moreon that in just a bit). These buttons, as created, don’t do anything. Whenclicked on by the user, they will behave as real buttons (depress, then release),but no action will occur. Yet.16.7.2.5 Actions for ButtonsWe need to attach an action to each button, which is little more than a specialclass to hold the code that you want to be run when the button is pressed. Wecan define the action as an anonymous inner class, so that the code is rightthere, inline with the rest of our code. Then we just attach that code to thebutton. Here is an example of that for our close button (the one labeled Quit):234 ActionListener closAction = new ActionListener()235 {236 public void237 actionPerformed(ActionEvent e)238 {239 System.exit(0);240 }241 } ;ActionListener is an interface—a very simple interface that defines justone method, actionPerformed(). You can take any class, have it extendActionListener, and then define an actionPerformed() method for it.That class can then serve as the action for a button. Here we just create an inlineclass that does nothing but the actionPerformed() method, and a prettysimple one at that. It simply exits.We could define the action elsewhere, and then just use the reference tothe action. If we had put the declaration of closAction at a higher lexicalscope (out at the beginning of the class definition, for example) then other UIelements could also use this action. Of course, if you’re going to share youraction between GUI elements, be sure that you write the code to be reentrant.Lines 244–267 (still within the createButtons()method) define theaction for the button labeled New Subaccount. Line 268 connects it to thebutton. Don’t pay attention to the specifics of this action just yet. We’ll discussit in detail below, once we know more about the other objects. Here is howthat action is built: