Details

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.*;

class ButtonFrame extends JFrame
{
  JButton bChange ; // reference to the button object

  // constructor for ButtonFrame
  ButtonFrame() 
  {
    // construct a Button
    bChange = new JButton("Click Me!");        3. A JButton is constructed. 

    // add the button to the JFrame
    getContentPane().add( bChange );           4. The JButton is added to the JFrame

                                               5. The default layout manager is used.
                                                  It puts one big button on the screen.
    
    setDefaultCloseOperation( WindowConstants.EXIT_ON_CLOSE );   6. The close button is set.
  }
}

public class ButtonDemo
{
  public static void main ( String[] args )    1. The program starts running with main().
  {
    ButtonFrame frm = new ButtonFrame();       2. main() constructs a ButtonFrame object. 

    frm.setSize( 200, 150 );                   7. The size of the frame is set. 
    frm.setVisible( true );                    8. The frame is made visible. 
  }
}

The ButtonFrame class does not to define a paint() method because everything in the frame is a Swing 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 need to override paint().

The program in the previous chapter did not define a constructor because the constructor it inherited from its parent class JFrame was enough to do the job. ButtonFrame adds a components, and this should be done in a constructor.