Java™ Application Development on Linux - Dator

352Chapter 16Getting in the Swing of Things: Designing a GUI for BudgetProGUI pieces—the status line, the table list of accounts, and the buttons. TheJFrame is a bit odd here, in that you have to add objects to its content pane;other container objects you can just add to directly. (We could have done thegetContentPane() once, store the result in an intermediate variable, and dothe adds to it, but the efficiency gain is unimportant here because we only needto do this once, to get the GUI started.)When we’ve got it built, we pack the frame, and make it visible:frame.pack();frame.setVisible(true);That’s the basic core of what you need to do with any GUI: construct itspieces, add them to the frame, pack the frame, and make it visible. Now you’reoff and running. The rest is just details.16.7.2 Creating PiecesThe three pieces that we create—the status, the list, and the buttons—will eachpackage up their objects into an intermediate container, a JPanel, and returnthat container to main(). This not only serves to chunk the problem intofewer pieces (just three parts, not eight or more), but also helps with theformatting. Each piece can format its objects relative to each other. Thenmain() only has to lay out the three big pieces. So watch for each of thecreate...() methods to return a JPanel—a good approach when you buildyour GUIs, too.The JPanels returned to main() are just Swing objects. They, like thebuttons or labels (that we will see here shortly), just get added into other containers.For main(), that container is the JFrame, the main window. Anycontainer will have a layout manager, the mechanism by which objects areplaced in that container. For JFrame, the default is the BorderLayout manager.When you call the add() method on a container using a BorderLayout,you can specify (as a second parameter to the add() method) where theobject being added will get placed. The constants defined for placingobjects are NORTH, SOUTH, EAST, WEST, or CENTER—hence the “Border” ofBorderLayout. There are also relative position values: PAGE_START,PAGE_END, LINE_START, and LINE_END which are just like north, south, west,and east, respectively, provided that the ComponentOrientation is set toLEFT_TO_RIGHT. (If you really want to know, check the Javadoc page forjava.awt.BorderLayout.)