Java Applets using Swing

							Java Applets using Swing

       Brian C. Hoffman
 IMMT and E-Business Conference
        June 26th, 2003
Goals and Objectives
   To understand the following terminology
      Java

      Object Oriented Programming

      Swing

      Java Applet

   To understand inheritance and the life cycle of a Java Applet
   To be able to write a simple Applet
   To understand how to insert Swing GUI components such as
    text boxes and buttons into an Applet
   To understand layout managers and panels
   To be able to handle events generated by GUI components
What is Java?
   Java is an object oriented language developed by James Gosling
    and coworkers at SUN Microsystems.

   Java began as a hardware independent language for powering
    small hand-held devices such as complex remotes.

   Java’s hardware independence made it ideal for use on the web
    and in 1994 the HotJava web browser was introduced. It was
    the first demo of interactive applets.

   In March 1995, it was announced that support for Java would
    be built into Netscape Navigator. And thus began the rapid
    growth of Java.
What does Object Oriented
Mean?
   An object is an instance of a class.
   A class is a programming construct that groups together data
    (properties) and functions (methods) that can be performed on
    that data.
   For example, consider representing a chair

     Properties                 Methods
         •Height                    •Create
         •Width                     •Destroy
         •Weight                    •Move-X
         •Color                     •Move-Y
         •X-Position                •Paint
         •Y-Position
Inheritance - A Powerful Tool
   Inheritance allows one to build a new class that builds onto the
    properties of an existing class.

   Models an “is a” relationship. For example, a faculty member is
    person who possesses some additional properties such as rank
    or number of publications.

   Inheritance keeps you from having to rewrite code. For
    example, Java applets are required to have over 200 methods
    but inheritance takes care of most of them.

   In Java inheritance is indicated by using the extends keyword
    What is Swing?
Swing is a collection of classes and interfaces
that one can use to display GUI components
or inherit from to create new components.


               A calculator written using Swing
       Advantages/Disadvantages of
       Swing
           Advantages                             Disadvantages
   Swing is the current standard for       Swing and Java 2 have limited
    writing GUI components in Java 2         built-in browser support. (Only
                                             Netscape 6.0+)
   Most Swing components are
    written, manipulated, and
    displayed purely in Java.               Plug-In for unsupported browsers
                                             is fairly large. You can obtain the
   Swing allows for a more uniform          plug-in from
    look and feel across platforms. It
    also allows for pluggable looks.         http://www.java.com/en/index.jsp

   Swing supports assistive
    technologies such as braille
    readers
What is a Java Applet
   Applets are Java programs that may be embedded
    into HTML documents. The applet runs in the page as
    soon as it has been downloaded.

   More specifically Java applets are Java programs that
    inherit the Applet (awt) or JApplet class (swing).

   For examples see http://javaboutique.internet.com/
Java Applet Framework
import java.awt.*;
import javax.swing.*;

public class NameApplet extends JApplet
{
  public void init()
  {
       // variable initializations, image loading, GUI setup, ect.
  }

    public void paint(Graphics g)
    {
      super.paint(g);
      // draw graphics
    }
}
Life Cycle of an Applet
The life cycle of an Applet consists of inherited functions that are called at various
times by the applet container (browser).

public void init()
 { // called when applet is first created }

public void start()
 { // called when applet is started and then restarted }

public void paint(Graphics G)
 { // invoked when screen needs to be drawn or redrawn }

public void stop()
 { //called when the user leaves the page containing the applet }

public void destroy()
 { // called when applet is permanently destroyed }
A “Hello World” Applet
import java.awt.*;
import javax.swing.*;

public class HelloApplet extends JApplet
{
  public void paint(Graphics g)
  {
   super.paint(g);
   // draw string at x=25 and y=25
   g.drawString(“Hello World”, 25, 25);
  }
}
Adding Applets to an HTML
Document
Insert the following where you wish the applet to appear:



<applet code=“MyApplet.class” height=“200” width=“100”>
  <param name=“” value=“”> </param>
  <param name=“” value=“”> </param>
  <param name=“” value=“”> </param>
</applet>
Other Graphics Methods to Try
   g.setFont( new Font(“Serif”, Font.BOLD, 12) )

g.setColor( new Color(0-255,0-255,0-255))


or g.setColor(Color.BLUE)

   g.drawLine(start_X, start_Y, end_X, end_Y)

   g.drawRect(topLeftX, topLeftY, width, height)

   g.fillRect(topLeftX, topLeftY, width, height)

   g.drawOval(x, y, width, height)

   g.fillOval(x, y, width, height)

For more methods see the Java API documentation at http://java.sun.com
Swing GUI Components
Swing provides assorted GUI components that you can add to a
JApplet. For example,

   JLabel

   JTextField

   JTextArea

   JButton

   JCheckBox

   JRadioButton
Inserting a GUI Component
To insert a Swing component you must declare it

JButton myButton = new JButton(“Button1”);

and then add it to the applet’s content pane as follows:

Container container = getContentPane();
container.add(myButton, BorderLayout.NORTH);

Where the GUI component goes depends upon the LayoutManager
used by the applet.
  The Border Layout Manager
     Layout within an applet is controlled by a Layout manager.
     The default manager is the BorderLayout Manager.




Positions are indicated by the constants BorderLayout.NORTH,
BorderLayout.WEST, BorderLayout.CENTER, etc…
A Simple Addition Applet
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class AdditionApplet extends JApplet
{
  // declare three text fields and a button
  JTextField numOne, numTwo, answer;
  JButton sumButton;

  // initialize the applet and create the GUI
  public void init()
  {

     // create the text fields
     numOne = new JTextField();
     numTwo = new JTextField();
     answer = new JTextField();

     // set the answer field to be uneditable and place default text in each field
     answer.setEditable(false);
     numOne.setText("Number1");
     numTwo.setText("Number2");
     answer.setText("Answer");
// create the button and add an actionListener to it
        JButton sumButton = new JButton("Compute Sum");
        sumButton.addActionListener( new SumListener() );


        // add all the GUI components to the appler
        Container container = getContentPane();
        container.add(numOne, BorderLayout.WEST);
        container.add(numTwo, BorderLayout.CENTER);
        container.add(answer, BorderLayout.EAST);
        container.add(sumButton, BorderLayout.SOUTH);
    }


    // create the private action listener class
    private class SumListener implements ActionListener
    {
        public void actionPerformed(ActionEvent event)
        {
            int a, b, sum;
            a = Integer.parseInt(numOne.getText());
            b = Integer.parseInt(numTwo.getText());
            sum=a+b;
            answer.setText( Integer.toString(sum));
        }
    } // end SumListener
}
    Event Handeling
   Before you can handle an event you must include the import statement

    import java.awt.event;

   Events are handeled by adding appropriate event listeners to the
    components you want to watch. For example,

    JButton sum;
    sum = new JButton(“Calculate Sum”);
    sum.addActionListener(new SumListener());

   Event listeners are objects created from a class (via new) that
    implements one of the Listener interfaces. For example,

    private class sumListener implements ActionListener
    { // code }
Event Handeling Cont.
   Each Listener interface requires that the class implement one or
    more methods that tells the computer what to do when events
    occurs. For example, an ActionListener requires one to write a
    actionPerformed method.

    public void actionPerformed (ActionEvent event)
    { // code to handle the event }

   Other event Listener interfaces include:
        ItemListener
        MouseListener
        MouseMotionListener
        KeyListener
Some Other Layout Managers
   To change the change the layout manager you use the
    setLayout command for the content area as follows:

    Container container = getContentPane();
    Container.setLayout( new LayoutManager());

   You replace LayoutManager() with the desired maganger. Some
    possibilities other than the BorderLayout include:
      FlowLayout() - no parameters needed

      GridLayout(rows, cols) - sets out a grid

      CardLayout() - lays components out like cards
Addition Applet Revisited
   Flow Layout
// make these changes to AdditionApplet
// add all the GUI components to the applet
Container container = getContentPane();
container.setLayout(new FlowLayout());
container.add(numOne);
container.add(numTwo);
container.add(answer);
container.add(sumButton);

   Grid Layout
// make these changes to AdditionApplet
// add all the GUI components to the applet
Container container = getContentPane();
container.setLayout(new GridLayout(4,1));
container.add(numOne);
container.add(numTwo);
container.add(answer);
container.add(sumButton);
Another Event Handeling
Example with CheckBoxes
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class CBoxApplet extends JApplet
{
  JCheckBox boldCheck;
  int fontStyle;

  // initialize the applet and create the GUI
  public void init()
  {
      // set the size of the applet
      setSize(400, 200);

      // create the check box and add an ItemListener to it
      boldCheck = new JCheckBox("Bold");
      boldCheck.addItemListener(new myBoxListener());

      // add everything to the applet
      Container container = getContentPane();
      container.add(boldCheck, BorderLayout.SOUTH);
  }
// draw graphics
  public void paint(Graphics g)
  {
      super.paint(g);
      g.setFont(new Font("SanSarif", fontStyle, 32));
      g.drawString("Java Applets are Cool", 10, 100);
  }


  // event handler
  private class myBoxListener implements ItemListener
  {
      int valBold;
      public void itemStateChanged(ItemEvent event)
      {
          if (event.getSource() == boldCheck)
          {
              if (event.getStateChange() == ItemEvent.SELECTED)
                valBold = Font.BOLD;
              else
                valBold = Font.PLAIN;
          }
          fontStyle = valBold;
          repaint();
      }
  }// end myBoxListener
}// end CBoxApplet
Panels
   Panels are containers that have their own layout managers and can be
    added to other containers such as a slots in the applet.

   To create a panel, simply declare it as follows:
    JPanel myPanel = new JPanel();

   To set its layout use the setLayout() method. For example,
    myPanel.setLayout(new GridLayout(1,5));

   To add GUI conponents to a panel, use the panel’s add method as
    follows
    myPanel.add(myButton);

   To add a panel to an applet, add it just like any other GUI component
    using the add method of the applet.
    container.add(MyPanel, BorderLayout.SOUTH);
Panels Cont.
   New types of panels that handle have their own components and
    handle their own events can be written by inheriting from JPanel.

    class myPanel extends JPanel
    {
       myPanel()
       { // constructor code such as setting layout and declaring and
         adding GUI components
       }
    }

   Panels use the paintComponent method instead of the paint method
    and it must be called to work either directly or through repaint().
Panels Example - Calculator
GUI
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class CalculatorGUIApplet extends JApplet
{
  // declare the button panel and buttons that make the calculator work
  private JTextField display;
  private JPanel buttonPanel;
  private JButton clrButton, rParenButton, lParenButton, divideButton;
  private JButton sevenButton, eightButton, nineButton, multButton;
  private JButton fourButton, fiveButton, sixButton, addButton;
  private JButton oneButton, twoButton, threeButton, minusButton;
  private JButton zeroButton, decimalButton, powButton, equalButton;

  // initialize the applet and create the GUI
  public void init()
  {
     // create the button pannel and set its layout manager
     buttonPanel = new JPanel();
     buttonPanel.setLayout(new GridLayout(5,4));
// build the buttom panel one row at a time
     // row 1
          clrButton=new JButton("Clr");
          buttonPanel.add(clrButton);
          lParenButton=new JButton("(");
          buttonPanel.add(lParenButton);
          rParenButton=new JButton(")");
          buttonPanel.add(rParenButton);
          divideButton=new JButton("/");
          buttonPanel.add(divideButton);

    // row 2
        sevenButton=new JButton("7");
        buttonPanel.add(sevenButton);
        eightButton=new JButton("8");
        buttonPanel.add(eightButton);
        nineButton=new JButton("9");
        buttonPanel.add(nineButton);
        multButton=new JButton("*");
        buttonPanel.add(multButton);
    …

     // create the display area
     display = new JTextField("0");
     display.setEditable(false);

    //add everything to the applet
    Container contentPane = getContentPane();
    contentPane.add(display, "North");
    contentPane.add(buttonPanel, "Center");
   }
}// end CBoxApplet
Simple Applet Ideas
   A calculator for a desired property

        Body Mass Index or Bone Density for Allied Health
        Unit Conversion or PV=nRT for Physics or Chemistry
        Interest Calculations for Business and Management
        Quadratic formula or Pythagorean Theorem for Math


   A quote of the Day Generator

    - create an array of strings (string [] quoteArray = new string[100];) holding the
    quotes and use a random number generator to see which string gets passed to
    drawString(). The code to generate the random number is:

    Random r = new Random(seedValue);
    index = r.nextInt() % quoteArray.length;
Bibliography
   H.M Deitel and P.J. Deitel. Java: How to Program 4th Edition.
       Prentise Hall; New Jersey. 2002.

   Marty Hall and Larry Brown. Core Web Programming 2nd Edition.
    Prentise Hall & Sun MicroSystems Press; New Jersey. 2001.

   Java 1.4.1 API documentation at http://java.sun.com/j2se/1.4.1/docs/api/