Creating a Frame Core Java

A top-level window (that is, a window that is not contained inside another window) is called a frame in Java. The AWT library has a class, called Frame, for this top level. The Swing version of this class is called JFrameand extends the Frame class. The JFrameis one of the few Swing components that is not painted on a canvas. Thus, the decorations (buttons, title bar, icons, and so on) are drawn by the user’s windowing system, not by Swing.

CAUTION: Most Swing component classes start with a “J”: JButton, JFrame, and so on. There are classes such as Button and Frame, but they are AWT components. If you accidentally omit a “J”, your program may still compile and run, but the mixture of Swing and AWT components can lead to visual and behavioral inconsistencies.

In this section, we go over the most common methods for working with a Swing JFrame.

Listing below lists a simple program that displays an empty frame on the screen, as illustrated in Figure below.

The simplest visible frame

The simplest visible frame

SimpleFrameTest.java

Let’s work through this program, line by line. The Swing classes are placed in the javax.swingpackage. The package name javaxindicates a Java extension package, not a core package. For historical reasons, Swing is considered an extension. However, it is present in every Java SE implementation since version 1.2.

By default, a frame has a rather useless size of 0 × 0 pixels. We define a subclass Simple - Frame whose constructor sets the size to 300 × 200 pixels. This is the only difference between a SimpleFrameand a JFrame.

In the main method of the SimpleFrameTestclass, we construct a Simple Frame object and make it visible. There are two technical issues that we need to address in every Swing program.

First, all Swing components must be configured from the event dispatch thread, the thread of control that passes events such as mouse clicks and keystrokes to the user interface components. The following code fragment is used to execute statements in the event dispatch thread:

NOTE: You will see many Swing programs that do not initialize the user interface in the event dispatch thread. It used to be perfectly acceptable to carry out the initialization in the main thread. Sadly, as Swing components got more complex, the programmers at Sun were no longer able to guarantee the safety of that approach. The probability of an error is extremely low, but you would not want to be one of the unlucky few who encounter an intermittent problem. It is better to do the right thing, even if the code looks rather mysterious.

Next, we define what should happen when the user closes the application’s frame. For particular program, we want the program to exit. To select this behavior, we use the statement

In other programs with multiple frames, you would not want the program to exit just because the user closes one of the frames. By default, a frame is hidden when the user closes it, but the program does not terminate. (It might have been nice if the program terminated after the last frame became invisible, but that is not how Swing works.)

Simply constructing a frame does not automatically display it. Frames start their life invisible. That gives the programmer the chance to add components into the frame before showing it for the first time. To show the frame, the main method calls the setVisiblemethod of the frame.

NOTE: Before Java SE 5.0, it was possible to use the show method that the JFrame class inherits from the superclass Window. The Window class has a superclass Component that also has a show method. The Component.showmethod was deprecated in Java SE 1.2. You are supposed to call setVisible(true) instead if you want to show a component. However, until Java SE 1.4, the Window.showmethod was not deprecated. In fact, it was quite useful, making the window visible and bringing it to the front. Sadly, that benefit was lost on the deprecation police, and

Java SE 5.0 deprecated the show method for windows as well.

After scheduling the initialization statements, the main method exits. Note that exiting main does not terminate the program, just the main thread. The event dispatch thread keeps the program alive until it is terminated, either by closing the frame or by calling the System.exitmethod.

The running program it is a truly boring top-level window. As you can see in the figure, the title bar and surrounding decorations, such as resize corners, are drawn by the operating system and not the Swing library. If you run the same program in Windows, GTK, or the Mac, the frame decorations are different.

The Swing library draws everything inside the frame. In this program, it just fills the frame with a default background color.

NOTE: As of Java SE 1.4, you can turn off all frame decorations by calling


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Core Java Topics