Java™ Application Development on Linux - Dator

17.3 Porting BudgetPro to SWT393• addKeyListener()• addMouseListener()• addMouseMoveListener()• addMouseTrackListener()• addPaintListener()• addTraverseListener()There are others. SWT naming conventions define an interface forwhich each add...Listener() method is named. For example, there is aSelectionListener interface. Many such interfaces have multiple methods,each to handle a distinct kind of event; for example, the MouseListener interfacedefines separate methods to handle a button down event, a button releaseevent, and a double-click event. As in Swing, it is common to implement eventlisteners as anonymous inner classes that implement the listener interface.However, since it is common to be interested only in some (or even only one)listener event, it is annoying to have to implement the full interface, since youhave to provide method implementations for every event. For this reason, SWTalso provides classes called adapters that implement “do-nothing” methods forevery listener event. These also follow a naming convention. For example, theadapter for the MouseListener interface is a class named MouseAdapter; theSelectionListener interface has an adapter named SelectionAdapter,and so on.For us, this means that we are going to create a reference to an anonymousinner class that implements the SelectionListener interface by extendingthe SelectionAdapter class. This is probably the weirdest common codeconstruct in Java. Let’s take a direct look at that method (Example 17.2).If you can correctly answer the following question, then you can be reasonablyassured that you do, in fact, understand what is going on here. Would theprogram compile and run correctly if the type of the upAction variable werechanged to SelectionAdapter? The answer is in the footnote. 1414. Yes, it would. The reason is that the addSelectionListener() method takes an argumentof type SelectionListener. Both SelectionListener and SelectionAdapterare of that base type. Aren’t Objects wonderful?