A Brief Introduction to Swing in Java

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is called an event loop Somewhere, there is an event loop that detects when the user presses a key, moves the mouse, or clicks a mouse button This detection is usually done at the system level, and not by the program As part of this process, the GUI tool kit will distinguish between different kind of events - a mouse click on a control, or a mouse motion over a drawing area, for example These events are then passed on to the program for appropriate action The exact method of handling the event-loop and passing the events on to the application code varies from toolkit to toolkit In Swing, the program doesn't need any direct access to the event-loop In Java, the event loop is really in a separate thread of execution which is started and controlled by the runtime system A Java program handles events using what are known as listeners and callbacks The section "Handling Swing Command Events" discusses events and Swing in more detail
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A Brief Introduction to Swing
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The original GUI toolkit provided by Java was called the AWT (Abstract Windowing Toolkit) While there are many existing Java AWT applications, the AWT has been superseded by the Swing toolkit Swing is generally more complete and easier to use than AWT It is almost too complete One of the problems of providing the GUI toolkit for a programming language is that it must provide everything, even if most of the features will be used by only a tiny fraction of programmers The myriad of details and options provided by Swing make it seem more complicated that it has to be We will try to focus on the essentials most Java programmers will need to use With the proper perspective, it is possible to get a handle on Swing in just a few pages So, let's use our general GUI model to look at how Swing can build applications that fit that model For the main application, Swing needs to provide the outer window that includes a title bar and special buttons, a menu bar, a tool bar, and place to hold the main view or views, often a drawing canvas While Swing has superseded the AWT, it is based upon and uses the AWT To distinguish similarly named classes in the AWT, Swing has adopted its own naming conventions For the most part, Swing classes start with a capital J, such as JFrame, JButton, or JSplitPane However, not all Swing classes start with a J (eg, Box or Timer) It is possible to mix AWT and Swing, but that really is not a good idea
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Swing provides the JFrame as the main class to support "standard" applications A JFrame is a top-level container that exists mainly to provide a place for other Swing components to paint themselves (Two other top-level components are also commonly used: JDialog for dialogs, and JApplet for applets) A JFrame will be displayed with the title frame, and
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has a single component called a JRootPane, which has a menu bar (a JMenuBar), a content pane (which is typically set to hold something like a general purpose JPanel by the user program), and some other things that generally don't matter for typical situations The fact that the JFrame uses a JRootPane isn't usually relevant for many practical situations It is really easy to create a simple Swing application with JFrame The sequence of code that will create the practical object structure shown in Figure 5-3 is given in Listing 5-1 Listing 5-1 Code excerpt
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// Setup a JFrame for practical use JFrame theFrame = new JFrame("Application Name"); JMenuBar theMenuBar = new JMenuBar(); // code to define items on Menu, event handlers, etc theFramesetJMenuBar(theMenuBar); JPanel thePanel = new JPanel(); // code to define what is on panel, layout manager, // define user interface, command control, etc // (non-trivial) theFramesetContentPane(thePanel); theFramepack(); theFramesetVisible(true); This code fragment would be found in the definition of some object This would usually be defined in the top-level class that has the main method, and defines the application It is likely that all this code would be found in the order shown here, but would have some code in between the steps to use the parts of the interface But the steps shown are really at the heart of the matter: 1 Create JFrame 2 Create and define JMenuBar 3 Add JMenuBar to JFrame 4 Create JPanel
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