• Example 1 - Drawing a Line
  
  
• Example 1 - Source Code

• Example 2 - Drawing a Rectangle
  
  
• Example 2 - Source Code
  
  
• Example 3 - Filling a Rectangle
  
  
• Example 3 - Source Code

• Example 4 - Drawing an Oval
  
  
• Example 4 - Source Code
  
  
• Example 5 - Drawing an Oval and its Bounding Box
  
  
• Example 5 - Source Code
  
  
• Example 6 - Drawing a Circle and its Bounding Box
  
  
• Example 6 - Source Code
  
  
• Example 7 - Filling an Oval
  
  
• Example 7 - Source Code
  
  
• Example 8 - Filling a Circle
  
  
• Example 8 - Source Code

• Example 9 - Drawing a Triangle
  
  
• Example 9 - Source Code

• Example 10 - Filling a Triangle
  
  
• Example 10 - Source Code

import java.awt.*;
import java.applet.*;

public class Example11 extends Applet implements Runnable {
   int x = 50;
   int y = 50;
   Thread example;

   public Example11() {
      setLayout(null);

      setBackground(Color.white);
   }

   public void init() {
      if (example == null) {
         example = new Thread(this);
         example.start();
      }          
   }

   public void run() {
      while (true) {
         x++;
         repaint();
         try {
            Thread.sleep(1000);
         } catch (InterruptedException exc) {
         }
         if (x==250)
            x=50;
      }
   }

   public void update(Graphics g) {
      System.out.println("Hello");

      g.setColor(getBackground());
      g.clearRect(x-1,y,50,50);
      g.setColor(getForeground());

      paint(g);
   }

   public void paint(Graphics g) {
      g.fillOval(x,y,50,50);  
   }

   public void destroy() {
      if (example != null) {
         example.stop();
         example = null;
      }
   }
 
}

1. In order to use applets, we must import the java.applet package.
2. We always extend the Applet class.
3. An apple cannot run by itself. It must be run in the context of a browser or appletviewer.
4. The first method executed when an applet is loaded is the constructor if there is one.
5. The next method executed is the init() method. The init() method is executed one time when the applet is loaded.
6. The next method executed is the start() method. The start() method is executed each time the applet becomes visible.
7. The stop() method is executed each time the applet becomes invisible, e.g. when the browser or appletviewer window is minimized.
8. The destroy() method is executed one time when the browser or appletviewer is closed.
9. Since an Applet is a Panel, we can break the applet up into smaller Panels, and in each of those Panels we can place valid Components.

1. Set the Layout manager for the applet to null.
2. Create the two Panel objects.
3. Decide on the size and location of those Panel objects.
4. Add the two Panel objects into the applet.

1. Create the Applet and implement the ActionListener interface.
2. Create a Button object, button, and give it a label, buttonLabel.
3. Tell the interpreter to listen for a user to click the button by adding the line button.addActionListener(this);.
4. Add the button into the appropriate Panel object.
5. Since we are implementing the ActionListener interface, we must implement the method public void actionPerformed(ActionEvent e);.
6. Within the actionPerformed method, we may access the button that caused the event to occur with either the method getSource() or the method getActionCommand(). getSource() returns an object, and getActionCommand() returns the string associated with the button. Recall that in order to use getSource() we need to make the Button object an instance variable.

1. Create a TextField component or TextArea component.
2. Remember that we need to make the component an instance variable if we wish to access it in the methods we implement.
3. Add the component into the appropriate field.
4. We can place text into the component by using the setText() method.
5. We can retrieve text from the component by using the getText() method.
6. Since we are retrieving a string from the component, if we wish to use it as a number we must convert it to a number. We can do this by using the valueOf method in the appropriate object in java.lang, e.g. Integer, Float, etc. After converting the string to the appropriate object we then call the method which will convert the number in the object to one of the primitive types. For example, to convert the text in TextField to an int,
   
   

   int value = (Integer.valueOf(myTextField.getText())).intValue();
   

1. Create the Applet and implement the ItemListener interface.
2. Create Checkbox objects, e.g. checkbox, and add them to the appropriate panel.
3. Add item listeners to each checkbox component with the line checkbox.addItemListener(this);.
4. Since we are implementing the ItemListener interface, we must implement the method public void itemStateChanged(ItemEvent e);.
5. When a box is checked or unchecked, an ItemEvent is created, and itemStateChanged is called.
6. We use the method getStateChange() to determine whether a box was checked or unchecked.
7. There are several static int variables defined in the ItemEvent class.
8. Two of these static ints are SELECTED and DESELECTED.
9. Once we have determined whether a box was checked or unchecked, we use the method getItem() to get the string value associated with the box that was checked or unchecked.
10. We may use the method getState() to determine whether a box is checked or unchecked. This method returns a boolean.
11. We may use the method setState(boolean b) to check a box or uncheck it.

1. In order to obtain a random number we use the method Random() in the class Math in java.lang. Random() returns a double between 0 and 1. In order to generate a random number between 0 and x, we multiply the return value by x. To generate a random integer between 0 and x, we cast the result as an int.
2. We use the concept of threads in order to accomplish having the figure on the screen for a random amount of time. We use the simple scheme of draw, wait, and redraw.

import java.awt.*;
import java.applet.*;
import java.util.*;

public class Exercise6 extends Applet {
   TextField timeLeft;
   Thread timer;

   public class Timer implements Runnable {
      int hours;
      int minutes;
      int seconds;
      Calendar time;
      int base;
      boolean first=true;
      int basehours;
      int baseminutes;
      int baseseconds;

      public void run() {
         timeLeft.setText("     ");

         while (true) {
            if (first) {
               time = Calendar.getInstance();

               basehours = time.get(Calendar.HOUR_OF_DAY);
               baseminutes = time.get(Calendar.MINUTE);
               baseseconds = time.get(Calendar.SECOND);

               System.out.println(basehours + " " + baseminutes + " " + baseseconds);

               base = basehours*60*60+baseminutes*60+baseseconds;

               System.out.println(base);

               first=false;
            }
            time = Calendar.getInstance();
            hours = time.get(Calendar.HOUR_OF_DAY);
            minutes = time.get(Calendar.MINUTE);
            seconds = time.get(Calendar.SECOND);
         
            int current = hours*60*60+minutes*60+seconds;
            
            timeLeft.setText(String.valueOf(301-(current-base)));

            try {
               Thread.sleep(1000);
            } catch (InterruptedException exc) {
               System.exit(1);
            }
         } 
      }
   }

   public Exercise6() {
      setLayout(new FlowLayout());

      timeLeft = new TextField("300");

      add(timeLeft);
   }

   public void init() {
      if (timer == null) {
         timer = new Thread(new Timer());

         timer.start();
      }
   }

   public void destroy() {
      timer.stop();
   }

}

1. Create the Applet and implement the MouseListener interface.
2. Tell the interpreter to listen for Mouse Events by adding the line addMouseListener(this) to the appropriate Panel.
3. Implement all the methods in the MouseListener interface, in particular mousePressed.
4. Use the getX() and getY() methods to get the coordinates of the mouse click relative to the upper left-hand corner of the Panel.

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

public class Exercise7 extends Applet implements MouseListener {
   Panel panel;

   public Exercise7() {
      setLayout(null);

      panel = new Panel(null);

      panel.setSize(500,300);
      panel.setLocation(0,0);

      panel.addMouseListener(this);

      add(panel);
   }

   public void mousePressed(MouseEvent e) {
      int x=0;
      int y;
 
      int x1 = e.getX();
      int y1 = e.getY();

      int xd=(250-x1)*(250-x1);
      int yd=(300-y1)*(300-y1);

      double diagonalDistance = Math.sqrt((250-x1)*(250-x1)+(300-y1)*(300-y1));
      int verticalDistance = Math.abs(300-y1);
      int horizontalDistance = Math.abs(250-x1);

      int horizontalDirection = 250-x1;

      int t = (int)(30*verticalDistance/diagonalDistance);
      int s = (int)(30*horizontalDistance/diagonalDistance);

      if (horizontalDirection<=0) {
         x = 250+s;
      } else if (horizontalDirection>0) {
         x = 250-s;
      }

      y = 300-t;

      Graphics g = panel.getGraphics();

      g.drawLine(250,300,x,y);

   }

   public void mouseClicked(MouseEvent e) {  }
   public void mouseReleased(MouseEvent e) {  }
   public void mouseEntered(MouseEvent e) {  }
   public void mouseExited(MouseEvent e) { }

}

1. Recall that in order for a figure to be hit, the mouse click must be in the direction of the figure, and there must be enough time for the bullet to get to the figure.

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

public class Exercise9 extends Applet implements ItemListener {
   Checkbox on;
   Checkbox off;
   TextField myTextField;

   public class MyException extends Exception {
      String s;

      public MyException(String s) {
         this.s = s;
      }
   }

   public Exercise9() {
      setLayout(null);

      Panel panel1 = new Panel(new FlowLayout());
      Panel panel2 = new Panel(new FlowLayout());

      myTextField = new TextField(100);

      panel1.add(myTextField);

      panel1.setSize(500,100);
      panel1.setLocation(0,0);

      on = new Checkbox("On");
      off = new Checkbox("Off");

      on.addItemListener(this);
      off.addItemListener(this);

      panel2.add(on);
      panel2.add(off);

      panel2.setSize(500,100);
      panel2.setLocation(0,100);

      add(panel1);
      add(panel2);
   }

   public void itemStateChanged(ItemEvent e) {
      if (e.getStateChange() == ItemEvent.SELECTED) {
         try {
            if (on.getState() && off.getState())
               throw new MyException("You can't check both boxes at the same time.");
            else if (on.getState() || off.getState())
               myTextField.setText("");            
         } catch (MyException exc) {
            myTextField.setText(exc.s);
            on.setState(false);
            off.setState(false);
         }
      }
   }

}