Java™ Application Development on Linux - Dator

344Chapter 16Getting in the Swing of Things: Designing a GUI for BudgetProSo what is left for you to do? You have to:• Construct the various items that will appear on the screen (in one or morewindows).• Specify the location of these objects in the windows (layout).• Provide snippets of code that are the actions associated with various events(events happen, for example, when a button is pressed or a mouse isclicked). These actions are the guts of the code that make your applicationbehave how you want.Remember, some behaviors are enforced by the GUI as part of standard“look and feel.” Some things, like layout, are up to you to do well so long asyou keep within the standard UI guidelines. And some is just specific to yourapplication.With that in mind, let’s walk through the “Hello, world” example for abrief description of what each line does.We begin with some import statements, to resolve references to bothSwing and AWT classes. Swing is built on top of AWT, so some of the classesthat you use will actually be AWT classes. The biggest difference this makes toyou is in the import statements.We then begin our class declaration, followed by the definition of the onlymethod in this class, main(). Swing applications will typically have othermethods, but for our simple example we only need this one.Now comes some real Swing. The creation of a JFrame object is very important.In Swing we need to have containers to hold the objects that we wantto display. The JFrame is a top-level container, one specifically meant to holdother objects, but also meant to be the first one of a containment hierarchy.(There are only three such top-level containers in Swing: JFrame, JDialog,and JApplet.)Next we create a JLabel. It’s a Swing object meant to hold small amountsof text. The text can be either constant or changing, but it’s not user-editable(that would be a different kind of Swing object).We add the label to the frame, so that when the window (the JFrame)appears, it will show our text inside.The setDefaultCloseAction() does what you think. When you pressthe X in the upper right of the window frame (or wherever your window managerputs it), then not only will the window go away, but the program will stoprunning. (This is not the standard default value since JFrames can contain