You want to create some GUI components and have them appear in a window.

Create a JFrame and add the components to its ContentPane.

The older Abstract Windowing Toolkit (AWT) had a simple frame component that allowed you to add components directly to it. “Good” programs always created a panel to fit inside the frame, and populated that. But some less-educated heathens often added components directly to the frame. The Swing JFrame is more complex; it comes with not one but two containers already constructed inside it. The ContentPane is the main container; you should normally use it as your JFrame’s main container. The GlassPane has a clear background and sits over the top of the ContentPane; its primary use is in temporarily painting something over top of the main ContentPane. Because of this, you need to use the JFrame’s getContentPane( ) method:

import java.awt.*;
import javax.swing.*;

public class ContentPane extends JFrame {
    public ContentPane(  ) {
        Container cp = getContentPane(  );
        // now add Components to "cp"...
    }
}

Then you can add any number of components (including containers) into this existing container, using the Container’s add( ) method:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/** Just a Frame
 */
public class JFrameDemo extends JFrame {
    boolean unsavedChanges = false;
    JButton quitButton;

    /** Construct the object including its GUI */
    public JFrameDemo(  ) {
        super("JFrameDemo");
        getContentPane(  ).add(quitButton = new JButton("Exit"));

        // These "action handlers" will be explained later in the chapter.
        quitButton.addActionListener(new ActionListener(  ) {
            public void actionPerformed(ActionEvent e) {
                setVisible(false);
                dispose(  );
                System.exit(0);
            }
        });
        addWindowListener(new WindowAdapter(  ) {
            public void windowClosing(WindowEvent e) {
                setVisible(false);
                dispose(  );
                System.exit(0);
            }
        });
            
        pack(  );
    }
}

This code compiles fine. But when we try to run it, of course, there is no main method. We need to create one:

public class JFrameDemoMain {
    // We need a main program to instantiate and show.
    public static void main(String[] args) {
        new JFrameDemo(  ).setVisible(true);
    }
}

Now we can run it and have it display. But there are two problems: it starts off tiny (on MS-Windows) or huge (on X Windows). And, when we do resize it, only the buttons show, and it always takes up the full size of the window. To solve these problems, we need to discuss layout management, a topic to which we now turn our attention.