Java™ Application Development on Linux - Dator

354Chapter 16Getting in the Swing of Things: Designing a GUI for BudgetProIt consists of three parts, one for the account name, one for the total valueof the account, and one for the remaining value. Each part will be representedby its own label, using a JLabel object. (We could have done the entire linein one label, but this gives us a few more objects to manipulate.) Since we wantto group the labels together, we create a JPanel, which is a Swing container,to hold all these objects. We’ll also add the JButton object (the variable namedupton).A JLabel is a simple Swing object. You can construct an empty one withnew JLabel(); but you can also construct a label with a String as its initialvalue, which is more useful. You can later change a label’s value with a call toits setText() method, as you see here from line 117:117 tot.setText("Total: $"+current.getTotal());16.7.2.2 FlowLayoutThe JLabels are added to their JPanel, but with no position argument, unlikethe JFrame and BorderLayout used in main(). JPanel has a different defaultlayout manager: It uses FlowLayout. With it, added objects are placed side byside according to the window size. If the window is narrowed, they will simplyflow onto the next line. (You won’t see this behavior if you narrow the Budget-Pro window, but that’s because the JPanel has been added to the JFrame’sNORTH region, which means it’s no longer just a FlowLayout that determinessizes.) FlowLayout is a layout that’s easy to use, but doesn’t give you muchcontrol; it was just fine for our purposes here.16.7.2.3 BoxLayoutAnother simple layout mechanism is the BoxLayout. It allows you to place theobjects like stacking boxes—though they can be stacked horizontally as well asvertically. Look at line 224:224 retval.setLayout(new BoxLayout(retval, BoxLayout.X_AXIS));Here we are creating a BoxLayout object and associating it with ourJFrame to manage its objects. When we create a BoxLayout we can tell it thatwe want to stack our objects horizontally (using either X_AXIS or LINE_AXIS)or vertically (using either Y_AXIS or PAGE_AXIS). Note that the BoxLayout