All the programs we've created until now have been applications.

			You can put an application with a GUI in a jar file and then double-click it to run it on your computer.

			An application has full access to all the resources on your computer.

			Applets are programs that run in a web browser like Internet Explorer or Firefox or Safari.

			Usually applets are stored on a server (remote computer) and when you give your browser a URL it loads a web page written in html (hypertext-markup-language) which, in turn, loads and runs the applet in the browser.

			To prevent an applet from stealing personal information, installing or running viruses or damaging your files in any way, applets run in a restricted "sandbox."

			As a Java programmer, you have an option of running an applet on your computer in a web browser or in Java's appletviewer.  Either way, you need an html file to load the applet.  BlueJ creates the necessary html file for you.

			Running an applet in BlueJ

			Sample applet code

			Sample applet code with MouseListener

			import javax.swing.*; import java.awt.*; /** * This class creates a panel that example shapes are * drawn on. */ public class DrawingPanel extends JPanel {    private JCheckBox[] checkBoxArray; // Check box array        /**     * Constructor     */           public DrawingPanel(JCheckBox[] cbArray)    {       // Reference the check box array.       checkBoxArray = cbArray;              // Set this panel's background color to white.       setBackground(Color.WHITE);              // Set the preferred size of the panel.       setPreferredSize(new Dimension(300, 200));    }        /**     * paintComponent method     */        public void paintComponent(Graphics g)    {       // Call the superclass paintComponent method.       super.paintComponent(g);              // Draw the selected shapes.       if (checkBoxArray[0].isSelected())       {          g.setColor(Color.BLACK);          g.drawLine(10, 10, 290, 190);       }       if (checkBoxArray[1].isSelected())       {          g.setColor(Color.BLACK);          g.drawRect(20, 20, 50, 50);       }       if (checkBoxArray[2].isSelected())       {          g.setColor(Color.RED);          g.fillRect(50, 30, 120, 120);       }       if (checkBoxArray[3].isSelected())       {          g.setColor(Color.BLACK);          g.drawOval(40, 155, 75, 50);       }         if (checkBoxArray[4].isSelected())       {          g.setColor(Color.BLUE);          g.fillOval(200, 125, 75, 50);       }       if (checkBoxArray[5].isSelected())       {          g.setColor(Color.BLACK);          g.drawArc(200, 40, 75, 50, 0, 90);       }       if (checkBoxArray[6].isSelected())       {          g.setColor(Color.GREEN);          g.fillArc(100, 155, 75, 50, 0, 90);       }    } }

			import javax.swing.*; import java.awt.*; import java.awt.event.*; /** * This class displays a drawing panel and a set of check * boxes that allow the user to select shapes. The selected * shapes are drawn on the drawing panel. */ public class GraphicsWindow extends JApplet {    // The following will reference an array of check boxes.    private JCheckBox[] checkBoxes;        // The titles array contains titles for the check boxes.    private String[] titles = { "Line", "Rectangle",                  "Filled Rectangle", "Oval", "Filled Oval",                  "Arc", "Filled Arc" };    // The following will reference a panel to contain    // the check boxes.                                   private JPanel checkBoxPanel;        // The following will reference an instance of the    // DrawingPanel class. This will be a panel to draw on.    private DrawingPanel drawingPanel;        /**     * init method     */           public void init()    {       // Build the check box panel.       buildCheckBoxPanel();              // Create the drawing panel.       drawingPanel = new DrawingPanel(checkBoxes);              // Add the check box panel to the east region and       // the drawing panel to the center region.       add(checkBoxPanel, BorderLayout.EAST);       add(drawingPanel, BorderLayout.CENTER);    }        /**     * The buildCheckBoxPanel method creates the array of     * check box components and adds them to a panel.     */        private void buildCheckBoxPanel()    {       // Create the panel.       checkBoxPanel = new JPanel();       checkBoxPanel.setLayout(new GridLayout(7, 1));       // Create the check box array.       checkBoxes = new JCheckBox[7];       // Create the check boxes and add them to the panel.       for (int i = 0; i < checkBoxes.length; i++)       {          checkBoxes[i] = new JCheckBox(titles[i]);          checkBoxes[i].addItemListener(new CheckBoxListener());          checkBoxPanel.add(checkBoxes[i]);       }    }    /**     * A private inner class that responds to changes in the     * state of the check boxes.     */        private class CheckBoxListener implements ItemListener    {       public void itemStateChanged(ItemEvent e)       {          drawingPanel.repaint();       }    } }

			import javax.swing.*; import java.awt.event.*; import java.awt.*; /** * This applet demonstrates how the mouse adapter * classes can be used. */ public class DrawBoxes2 extends JApplet {    private int currentX = 0; // Current X coordinate    private int currentY = 0; // Current Y coordinate    private int width = 0;    // Rectangle width    private int height = 0;   // Rectangle height    /**     * init method     */    public void init()    {       // Add a mouse listener and a mouse motion listener.       addMouseListener(new MyMouseListener());       addMouseMotionListener(new MyMouseMotionListener());    }        /**     * paint method     */    public void paint(Graphics g)    {       // Call the superclass's paint method.       super.paint(g);              // Draw a rectangle.       g.drawRect(currentX, currentY, width, height);    }        /**     * Mouse listener class     */    private class MyMouseListener extends MouseAdapter    {       public void mousePressed(MouseEvent e)       {          // Get the mouse cursor's X and Y coordinates.          currentX = e.getX();          currentY = e.getY();       }    }        /**     * Mouse Motion listener class     */    private class MyMouseMotionListener extends MouseMotionAdapter    {       public void mouseDragged(MouseEvent e)       {          // Calculate the size of the rectangle.          width = e.getX() - currentX;          height = e.getY() - currentY;                    // Repaint the window.          repaint();       }    } }

			Replace public class Calc with public class CalcApplet extends JApplet

			Replace public static void main(String[] args) with public void init()

			Replace Calc calc = new Calc(); with CalcApplet calc = new CalcApplet();

			In first line of constructor, call constructer of the parent class:  super();

			Remove setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

			// ColorMatch.java import java.awt.*; import java.awt.event.*; import javax.swing.*; import javax.swing.border.*; import javax.swing.event.*; import java.util.*; public class ColorMatch {     static final int WIN_WIDTH = 500;     static final int WIN_HEIGHT = 400;     static final int MAX_RGB = 256;     private JFrame window;     private Random r = null;     private int randomRed;     private int randomGreen;     private int randomBlue;     private int userRed = 0;     private int userGreen = 0;     private int userBlue = 0;     private JPanel randomPanel = null;     private JPanel userPanel = null;     private JSlider redSlider = null;     private JSlider greenSlider = null;     private JSlider blueSlider = null;     private JButton showValues = null;     private JButton reset = null;          public static void main(String args[]) {         ColorMatch cm = new ColorMatch();     }          public ColorMatch() {           window = new JFrame("Color Match");              window.setSize(WIN_WIDTH,WIN_HEIGHT);         window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);         window.setLayout(new BorderLayout());         JPanel panBack = new JPanel(new BorderLayout());         r = new Random(System.currentTimeMillis());         Box hBox = Box.createHorizontalBox();         randomPanel = new JPanel();         randomRed = r.nextInt(MAX_RGB);         randomGreen = r.nextInt(MAX_RGB);         randomBlue = r.nextInt(MAX_RGB);         randomPanel.setBackground(new Color(randomRed,randomGreen,randomBlue));         randomPanel.setBorder(new EtchedBorder());         hBox.add(randomPanel);                  userPanel = new JPanel();         userPanel.setBackground(new Color(userRed,userGreen,userBlue));         userPanel.setBorder(new EtchedBorder());         hBox.add(userPanel);         panBack.add(hBox,BorderLayout.CENTER);                  JPanel panControls = new JPanel(new GridLayout(2,3));// sliders         JLabel jlr = new JLabel("Red",SwingConstants.CENTER);         JLabel jlg = new JLabel("Green",SwingConstants.CENTER);         JLabel jlb = new JLabel("Blue",SwingConstants.CENTER);         redSlider = new JSlider(SwingConstants.HORIZONTAL,0,MAX_RGB-1,0);         redSlider.setPaintLabels(true);         redSlider.setMajorTickSpacing(50);         redSlider.setPaintTicks(true);         greenSlider = new JSlider(SwingConstants.HORIZONTAL,0,MAX_RGB-1,0);         greenSlider.setPaintLabels(true);         greenSlider.setMajorTickSpacing(50);         greenSlider.setPaintTicks(true);         blueSlider = new JSlider(SwingConstants.HORIZONTAL,0,MAX_RGB-1,0);         blueSlider.setPaintLabels(true);         blueSlider.setMajorTickSpacing(50);         blueSlider.setPaintTicks(true);         panControls.add(jlr);         panControls.add(jlg);         panControls.add(jlb);         panControls.add(redSlider);         panControls.add(greenSlider);         panControls.add(blueSlider);         panBack.add(panControls,BorderLayout.SOUTH);                  window.add(panBack,BorderLayout.CENTER);                 JPanel panButtons = new JPanel();             panButtons.setLayout(new GridLayout(1,2));         showValues = new JButton("Show Values");         reset = new JButton("Reset");         BevelBorder edge = new BevelBorder(BevelBorder.RAISED);         showValues.setBorder(edge);         reset.setBorder(edge);         panButtons.add(showValues);         panButtons.add(reset);         window.add(panButtons,BorderLayout.SOUTH);                  redSlider.addChangeListener(new ChangeListener()  // listeners         {             public void stateChanged(ChangeEvent e)             {                 userRed = redSlider.getValue();                 userPanel.setBackground(new Color(userRed,userGreen,userBlue));             }         });                  greenSlider.addChangeListener(new ChangeListener()         {             public void stateChanged(ChangeEvent e)             {                 userGreen = greenSlider.getValue();                 userPanel.setBackground(new Color(userRed,userGreen,userBlue));             }         });                  blueSlider.addChangeListener(new ChangeListener()         {             public void stateChanged(ChangeEvent e)             {                 userBlue = blueSlider.getValue();                 userPanel.setBackground(new Color(userRed,userGreen,userBlue));             }         });                          showValues.addActionListener(new ActionListener()         {             public void actionPerformed(ActionEvent e)             {                 String s1 = "Red:    " + format(randomRed,3);                 String s2 = "Red:   " + format(userRed,3);                 String s3 = "Green:  " + format(randomGreen,3);                 String s4 = "Green: " + format(userGreen,3);                 String s5 = "Blue:   " + format(randomBlue,3);                 String s6 = "Blue:  " + format(userBlue,3);                 String s = "     ";                 String n = "\n";                 JOptionPane.showMessageDialog(null,s1+s+s2+n+s3+s+s4+n+s5+s+s6);             }         });                  reset.addActionListener(new ActionListener()         {             public void actionPerformed(ActionEvent e)             {                 randomRed = r.nextInt(MAX_RGB);                 randomGreen = r.nextInt(MAX_RGB);                 randomBlue = r.nextInt(MAX_RGB);                 randomPanel.setBackground(new Color(randomRed,randomGreen,randomBlue));                 userRed = 0;                 userGreen = 0;                 userBlue = 0;                 userPanel.setBackground(new Color(userRed,userGreen,userBlue));                 redSlider.setValue(userRed);                 greenSlider.setValue(userGreen);                 blueSlider.setValue(userBlue);             }         });         window.setVisible(true);     }     public String format(int value, int field) {         String s = (new Integer(value)).toString();         while (s.length() < field) {             s = " " + s;         }         return s;     } }