First please click on the button labelled 1 and observe the display start to form. Please note that the button labelled 1 will have the focus. In Internet Explorer 5 this is indicated by a dotted line inside the edge of the button as shown in the illustration.

The yellow and cyan pixels are being drawn on the screen. Please note the dotted line inside the edge of the button labelled 1 showing that it has the focus.

I have only tried this program using Internet Explorer 5 and other systems may react differently. While the drawing is taking place down the screen, please click on the button labelled 2. Nothing happens. When the square display area has completed drawing, the white square will be drawn on the screen.

The white square has been drawn on the screen.

Now, please refresh the program and click on the button labelled 1. While the drawing is taking place down the screen, please click on the button labelled 2 and then on the button labelled 3. When the square display area has completed drawing, the green disc will be drawn on the screen, but the white square will not be drawn.

The green disc has been drawn on the screen.

Adding some checkboxes.

The program is now adapted to add two checkboxes. The scenario used is that with the checkboxes set unchecked, that is in the false state, the program will continue to act as before. One checkbox, labelled magenta, will be used to control a boolean variable named usemagentainsteadofyellow and another checkbox, labelled blue, will be used to control a boolean variable named useblueinsteadofcyan. However, the checkbox labelled magenta will be active and the checkbox labelled blue will be passive. Active and passive as used here are my own parlance, they might not be used elsewhere in this context. The active checkbox will have a listener and will respond to an event. The passive checkbox will not have a listener and will not respond to an event, other than the box becoming ticked or unticked as may be the case.

Please note the addition of the word ItemListener shortly after the word implements and the addition of the itemStateChanged function. There are also other additions to the program including declaring the checkboxes and two boolean varaibles, locating and sizing the checkboxes and adding a listener to one of them; together with changes to the paint function.

import java.awt.*;
import java.awt.event.*;

public class Applet4 extends java.applet.Applet
                             implements ActionListener,
                                        ItemListener
  {
    int obeycode = 1;
    int xzero = 140;
    int yzero = 40;
    Button button51 = new Button("1");
    Button button52 = new Button("2");
    Button button53 = new Button("3");
    Checkbox checkbox71 = new Checkbox("magenta");
    Checkbox checkbox72 = new Checkbox("blue");
    boolean usemagentainsteadofyellow = false;
    boolean useblueinsteadofcyan = false;

    public void init()
      {
        setBackground(Color.red);
        setLayout(null);
        setBounds(0,0,500,300);
        button51.addActionListener(this);
        button51.setLocation(20,20);
        button51.setSize(100,30);
        add(button51);
        button52.addActionListener(this);
        button52.setLocation(20,70);
        button52.setSize(100,30);
        add(button52);
        button53.addActionListener(this);
        button53.setLocation(20,120);
        button53.setSize(100,30);
        add(button53);
        checkbox71.addItemListener(this);
        checkbox71.setLocation(20,170);
        checkbox71.setSize(100,30);
        add(checkbox71);
//      checkbox72 deliberately has no addItemListener
        checkbox72.setLocation(20,220);
        checkbox72.setSize(100,30);
        add(checkbox72);  
      }

    public void actionPerformed(ActionEvent event)
      {
        Object source = event.getSource();
        if (source == button51)
          {
            obeycode=51;
          }
        if (source == button52)
          {
            obeycode=52;
          }
        if (source == button53)
          {
            obeycode=53;
          }
        repaint();
      }

    public void itemStateChanged(ItemEvent event)
      {
        Object source = event.getSource();
        if (source == checkbox71)
          {
            obeycode=71;
            usemagentainsteadofyellow = checkbox71.getState();
          }
//      repaint is deliberately not called
      }

    public void paint(Graphics screen)
      {
        if (obeycode == 51)
          {
            for(int y=0; y <= 219; y++)
              {
                for(int x=0; x <= 219; x++)
                  {
                    double achieved = 0.0;
                    double target = 0.0;
                    int number = 2000;
                    for(int i=1; i <= number; i++)
                      {
                        achieved=achieved+Math.random();
                        target=target + 0.5;
                      }
                    if (achieved >= target)
                      {
                        if (usemagentainsteadofyellow)
                          {
                            screen.setColor(Color.magenta);
                          }
                        else
                          {
                            screen.setColor(Color.yellow);
                          }
                      }
                    else
                      {
                        useblueinsteadofcyan=checkbox72.getState();
                        if (useblueinsteadofcyan)
                          {
                            screen.setColor(Color.blue);
                          }
                        else
                          {
                            screen.setColor(Color.cyan);
                          }
                      }
                    screen.drawLine(xzero+x,yzero+y,
                                      xzero+x,yzero+y);
                  }
              }
          }

        if (obeycode == 52)
          {
            screen.setColor(Color.white);
            screen.fillRect(400,20,40,40);
          }

        if (obeycode == 53)
          {
            screen.setColor(Color.green);
            screen.fillOval(400,80,40,40);
          }
      }

    public void update(Graphics screen)
      {
        paint(screen);
      }

  }