Java™ Application Development on Linux - Dator

388Chapter 17Other Ways: Alternatives to SwingThe next step is to convert the GUI members of the class from the Swingclasses to their SWT counterparts. Of course, SWT requires the Display class,which has no analog in SWT, so we add a Display type member named dispjust ahead of the frame member.Next, we change the type of frame from JFrame to Shell. We could renamethe member, 10 but why add to our typing burden? The name is still clearand meaningful, even if it doesn’t match the SWT name. 11 There’s more to itthan just changing the type, however. The constructor call for the JFramedoesn’t match any constructor for Shell. In fact, the Shell constructor requiresa Display object argument, and all subsequent constructors for widgetsand controls require a Composite as an argument.This is a key difference between Swing and SWT. Swing allows you tobuild GUI components in arbitrary order at arbitrary times and then join themup to the GUI with an add() method call. SWT instead requires that you linkyour components up to the GUI element they belong to when they are constructed.There are good reasons for this difference. Remember that SWT allocatesnative objects and memory that Java’s garbage collector cannot recover. Becauseof this, SWT makes the promise that if you call the dispose() method on anySWT object, it will dispose of it and everything it contains. That allows you toclean up all resources from an SWT program by calling dispose() on the toplevel Display object. If SWT allowed you to build GUI structures independentlyand then graft them onto the hierarchy, it could not keep this promise.For this reason (amongst others) SWT objects are always built in a fairly rigidtop-down manner. 12The most direct consequence of this is that we have to get rid of the constructorson these declarations. We’ll start construction in the main(). So, awaywith the constructors for the GUI elements. We now need to change theJButtons to Buttons and the JLabels to Labels. Again, if you are using adynamic IDE, you should see your error count skyrocket with these changes10. If you are using Eclipse, this is easily done throughout your code with the Refactoringfeature.11. All right, I’m being lazy. Write your own book if you don’t like it.12. In some ways, this greatly simplifies SWT programs, but at the cost of some reusability.With Swing, you could construct a panel or other GUI element and reuse it in many places.You can achieve the same thing in SWT by encapsulating such a construct in its own class andpassing in a parent to the constructor, but this is a bit more bulky and complex than theSwing way.