Details

There is a listener for the frame's "close button," but not for the button. You can click the button, and generate an event, but no listener receives the event.

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

public class ButtonDemo extends JFrame
{
  JButton bChange ; 

  // constructor for ButtonDemo
  ButtonDemo() 
  {
    // construct a Button
    JButton bChange = new JButton("Click Me!"); 

    // add the button to the JFrame
    getContentPane().add( bChange );     
  }

  public static void main ( String[] args )
  {
    ButtonDemo frm = new ButtonDemo();
    
    WindowQuitter wquit = new WindowQuitter();
    frm.addWindowListener( wquit );
    
    frm.setSize( 200, 150 );     
    frm.setVisible( true );      
  }
}

class WindowQuitter  extends WindowAdapter
{
  public void windowClosing( WindowEvent e )
  {
    System.exit( 0 );  
  }
}

• The program starts running with main().
• main() asks the system to construct a buttonDemo object.
• A JButton is constructed using new with a constructor, as with any class.
  • The words on the button will be "Click Me!"
  • The variable bChange refers to the JButton.
• The buttonDemo constructor uses getContentPane().add( bChange ) to add the button to the frame.
• The default layout is used in adding the button to the frame.
  • The default is one big button in the center.
• The buttonDemo class does not have its own paint() method because everything to be painted is a component.
• The system will automatically paint all components in a container when it needs to.
• If special processing is needed, (as with drawString() in the previous chapter), then you will need to override paint().

The program in the previous chapter does not define a constructor because it inherits the constructor from its parent class (JFrame), which is enough to do the job. When you add components to a container, you need to define a constructor for the frame.