A Simple Swing Application

A Simple Swing Application

Swing programs differ from both the console-based programs and the AWT-based programs shown earlier in this book. For example, they use a different set of components and a different container hierarchy than does the AWT. Swing programs also have special requirements that relate to threading. The best way to understand the structure of a Swing program is to work through an example. There are two types of Java programs in which Swing is typically used. The first is a desktop application. The second is the applet. This section shows how to create

a Swing application. The creation of a Swing applet is described later in this chapter. Although quite short, the following program shows one way to write a Swing

application. In the process, it demonstrates several key features of Swing. It uses two Swing components: JFrame and JLabel. JFrame is the top-level container that is commonly used for Swing applications. JLabel is the Swing component that creates a label, which is a component that displays information. The label is Swing’s simplest component because

it is passive. That is, a label does not respond to user input. It just displays output. The program uses a JFrame container to hold an instance of a JLabel. The label displays a short text message.

// A simple Swing application.

import javax.swing.*;

class SwingDemo {

SwingDemo() {

// Create a new JFrame container.

JFrame jfrm = new JFrame("A Simple Swing Application");

     Give the frame an initial size. jfrm.setSize(275, 100);

     Terminate the program when the user closes the application. jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

     Create a text-based label.

JLabel jlab = new JLabel(" Swing means powerful GUIs.");

     Add the label to the content pane. jfrm.add(jlab);

     Display the frame. jfrm.setVisible(true);

}

public static void main(String args[]) {

// Create the frame on the event dispatching thread.

SwingUtilities.invokeLater(new Runnable() {

public void run() { new SwingDemo();

}

});

}

}

Swing programs are compiled and run in the same way as other Java applications. Thus, to compile this program, you can use this command line:

javac SwingDemo.java

To run the program, use this command line:

java SwingDemo

When the program is run, it will produce a window similar to that shown in Figure 31-1. Because the SwingDemo program illustrates several core Swing concepts, we will

examine it carefully, line by line. The program begins by importing javax.swing. As mentioned, this package contains the components and models defined by Swing. For example, javax.swing defines classes that implement labels, buttons, text controls, and menus. It will be included in all programs that use Swing.

Figure 31-1   The window produced by the SwingDemo program

Next, the program declares the SwingDemo class and a constructor for that class. The constructor is where most of the action of the program occurs. It begins by creating a JFrame, using this line of code:

JFrame jfrm = new JFrame("A Simple Swing Application");

This creates a container called jfrm that defines a rectangular window complete with a title bar; close, minimize, maximize, and restore buttons; and a system menu. Thus, it creates a standard, top-level window. The title of the window is passed to the constructor.

Next, the window is sized using this statement:

jfrm.setSize(275, 100);

The setSize( ) method (which is inherited by JFrame from the AWT class Component) sets the dimensions of the window, which are specified in pixels. Its general form is shown here:

void setSize(int width, int height)

In this example, the width of the window is set to 275 and the height is set to 100.

By default, when a top-level window is closed (such as when the user clicks the close box), the window is removed from the screen, but the application is not terminated. While this default behavior is useful in some situations, it is not what is needed for most applications. Instead, you will usually want the entire application to terminate when its top-level window is closed. There are a couple of ways to achieve this. The easiest way is to call setDefaultCloseOperation( ), as the program does:

jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

After this call executes, closing the window causes the entire application to terminate. The general form of setDefaultCloseOperation( ) is shown here:

void setDefaultCloseOperation(int what)

The value passed in what determines what happens when the window is closed. There are several other options in addition to JFrame.EXIT_ON_CLOSE. They are shown here:

DISPOSE_ON_CLOSE

HIDE_ON_CLOSE

DO_NOTHING_ON_CLOSE

Their names reflect their actions. These constants are declared in WindowConstants, which is an interface declared in javax.swing that is implemented by JFrame.

The next line of code creates a Swing JLabel component:

JLabel jlab = new JLabel(" Swing means powerful GUIs.");

JLabel is the simplest and easiest-to-use component because it does not accept user input. It simply displays information, which can consist of text, an icon, or a combination of the two. The label created by the program contains only text, which is passed to its constructor.

The next line of code adds the label to the content pane of the frame:

jfrm.add(jlab);

As explained earlier, all top-level containers have a content pane in which components are stored. Thus, to add a component to a frame, you must add it to the frame’s content pane. This is accomplished by calling add( ) on the JFrame reference (jfrm in this case). The general form of add( ) is shown here:

Component add(Component comp)

The add( ) method is inherited by JFrame from the AWT class Container.

By default, the content pane associated with a JFrame uses border layout. The version of add( ) just shown adds the label to the center location. Other versions of add( ) enable you to specify one of the border regions. When a component is added to the center, its size is adjusted automatically to fit the size of the center.

Before continuing, an important historical point needs to be made. Prior to JDK 5, when adding a component to the content pane, you could not invoke the add( ) method directly on a JFrame instance. Instead, you needed to call add( ) on the content pane of the JFrame object. The content pane can be obtained by calling getContentPane( ) on a JFrame instance. The getContentPane( ) method is shown here:

Container getContentPane( )

It returns a Container reference to the content pane. The add( ) method was then called on that reference to add a component to a content pane. Thus, in the past, you had to use the following statement to add jlab to jfrm:

jfrm.getContentPane().add(jlab); // old-style

Here, getContentPane( ) first obtains a reference to content pane, and then add( ) adds the component to the container linked to this pane. This same procedure was also required to invoke remove( ) to remove a component and setLayout( ) to set the layout manager for the content pane. You will see explicit calls to getContentPane( ) frequently throughout pre-5.0 code. Today, the use of getContentPane( ) is no longer necessary. You can simply call add( ), remove( ), and setLayout( ) directly on JFrame because these methods have been changed so that they operate on the content pane automatically.

The last statement in the SwingDemo constructor causes the window to become visible:

jfrm.setVisible(true);

The setVisible( ) method is inherited from the AWT Component class. If its argument is true, the window will be displayed. Otherwise, it will be hidden. By default, a JFrame is invisible, so setVisible(true) must be called to show it.

Inside main( ), a SwingDemo object is created, which causes the window and the label to be displayed. Notice that the SwingDemo constructor is invoked using these lines of code:

SwingUtilities.invokeLater(new Runnable() { public void run() {

new SwingDemo();

}

});

This sequence causes a SwingDemo object to be created on the event dispatching thread rather than on the main thread of the application. Here’s why. In general, Swing programs are event-driven. For example, when a user interacts with a component, an event is generated. An event is passed to the application by calling an event handler defined by the application. However, the handler is executed on the event dispatching thread provided by Swing and not on the main thread of the application. Thus, although event handlers are defined by your program, they are called on a thread that was not created by your program.

To avoid problems (including the potential for deadlock), all Swing GUI components must be created and updated from the event dispatching thread, not the main thread of the application. However, main( ) is executed on the main thread. Thus, main( ) cannot directly instantiate a SwingDemo object. Instead, it must create a Runnable object that executes on the event dispatching thread and have this object create the GUI.

To enable the GUI code to be created on the event dispatching thread, you must use one of two methods that are defined by the SwingUtilities class. These methods are invokeLater( ) and invokeAndWait( ). They are shown here:

static void invokeLater(Runnable obj)

static void invokeAndWait(Runnable obj)

throws InterruptedException, InvocationTargetException

Here, obj is a Runnable object that will have its run( ) method called by the event dispatching thread. The difference between the two methods is that invokeLater( ) returns immediately, but invokeAndWait( ) waits until obj.run( ) returns. You can use one of these methods to call a method that constructs the GUI for your Swing application, or whenever you need to modify the state of the GUI from code not executed by the event dispatching thread. You will normally want to use invokeLater( ), as the preceding program does. However, when constructing the initial GUI for an applet, you will need to use invokeAndWait( ).