Java Applet

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Java Applet

• Lin, Cui
• Computer Science Department
• Wayne State University

2
What is Applet

• A small program written in Java and included in a
  HTML page. It is independent of the operating
  system on which it runs
• A small computer program that can be transmitted
  from a server to a client computer and executed
  on the client.
• A small program that can be sent along with a Web
  page to a user. Java applets can perform
  interactive animations, immediate calculations,
  or other simple tasks without having to send a
  user request back to the server.

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Features of Applet

• Run inside web browser or AppletViewer
• OOP programming
• Graphical Java programs
• Platform-neutral

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Applet and HTML

• An applet is accessed by an HTML file.
• HTML is a mark-up language (it adds to text
  content by marking it up with tags)
• Browsers display the text and process the tags
  For example
• CIS255 ltbgt Homework lt/bgt Assignment
• The browser would display
• CIS255 Homework Assignment

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Applet and HTML

• CrossWord Example
• ltHTMLgt
• ltHEADgt
• ltTITLEgtCrossWordlt/TITLEgt
• lt/HEADgt
• ltBODYgt
• ltCENTERgt
• ltAPPLET codebase"classes" code"CrossWord.class"
  width960 height600gtlt/APPLETgt
• lt/CENTERgt
• lt/BODYgt
• lt/HTMLgt

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How Applet works
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The Applet Class (Overview)

• The Java class Applet contains all the behaviors
  (methods), which an applet container (browser)
  expects to see. (e.g. init, start, paint, stop,
  destroy)
• When creating an applet class, your class extends
  the Applet class, and inherits these behaviors.
• Your class can then override any of the behaviors

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The Applet Class (Basic operations)
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The Applet Class (Paint)

• System-triggered Paintingthe system requests a
  component to render its contents, usually for one
  of the following reasons
• The component is first made visible on the
  screen.
• The component is resized.
• The component has damage that needs to be
  repaired.
• App-triggered Painting
• the component decides it needs to update its
  contents because its internal state has changed.

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The Applet Class (Paint)
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The Applet Class (Repaint)

• The program determines that either part or all of
  a component needs to be repainted in response to
  some internal state change.
• The program invokes repaint() on the component,
  which registers an asynchronous request to the
  AWT that this component needs to be repainted.

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The Applet Class (Repaint)

• public void Solve()
• for (int i 0 i lt BLKSDOWN i)
• for (int j 0 j lt BLKSACROSS j)
• guessesij Character.toUpperCase(
  answersij)
• repaint()
• public void Clear()
• for (int i 0 i lt BLKSDOWN i)
• for (int j 0 j lt BLKSACROSS j)
• guessesij ' '
• repaint()

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The Applet Class (Update)

• When the applet invokes repaint(), to request its
  drawing area be repainted, AWT (Abstract Window
  Toolkit) invokes update().
• The inherited update() method clears the applet's
  drawing area to its background color before
  invoking paint().

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The Applet Class (Update)
public void update(Graphics g) Image
backbuffer Graphics backg
Font numFont new Font("Arial", Font.PLAIN,
10 ) Font letFont new Font("Arial",
Font.BOLD, 12 ) backbuffer
createImage( WIDTH5, HEIGHT5) backg
backbuffer.getGraphics()
backg.setColor(Color.WHITE)
backg.setFont(letFont)..
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The Applet Class (Event Listener)

• Every time the user types a character or pushes a
  mouse button, an event occurs.
• Any object can be notified of the event.
• All the object has to do is implement the
  appropriate interface and be registered as an
  event listener on the appropriate event source.

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The Applet Class (Event Listener)

• In the declaration for the event handler class,
  one line of code specifies that the class either
  implements a listener interface or extends a
  class that implements a listener interface.
  public class MyClass implements ActionListener
  
• Another line of code registers an instance of the
  event handler class as a listener on one or more
  components. someComponent.addActionListener(inst
  anceOfMyClass)
• The event handler class has code that implements
  the methods in the listener interface. public
  void actionPerformed(ActionEvent e) ...//code
  that reacts to the action...

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The Applet Class (Event Listener)
public void mouseClicked(MouseEvent e)
int i (int)(e.getY() / (1.0
HEIGHT/BLKSDOWN)) public
void mousePressed(MouseEvent e)
public void mouseReleased(MouseEvent e)
public void mouseEntered(MouseEvent e)
public void mouseExited(MouseEvent e)
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Procedures of creating an Applet

• Develop Java source code to implement your applet
  
• Compile the source file to produce class file
• Make an HTML file with the applet tag that
  references that class file
• Use browser to execute that HTML file OR run
  AppletViewer utility.

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Implement Applet (6-1)

• Every applet must implement one or more of the
  init, start, and paint methods
• The first lines must import the necessary classes
  used in the applet

import java.io. import java.lang.String import
java.lang.System import javax.swing. import
javax.swing.event. import java.awt.event. impo
rt java.awt. ...
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Implement Applet (6-2)

• Important linkshttp//java.sun.com/downloads/
  (JDK and JRE)http//java.sun.com/j2se/1.4.2/docs
  /api/index.html (Applet, Paint, Repaint,
  Update)http//java.sun.com/docs/books/tutorial/u
  iswing/events/generalrules.html(Listener and
  Event)

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Implement Applet (6-3)
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Implement Applet (6-4)

• An example of good program// CheckerDrag.java
• import java.awt.
• import java.awt.event.
• public class CheckerDrag extends
  java.applet.Applet
• // Dimension of checkerboard square.
• final static int SQUAREDIM 40

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Implement Applet (6-5)
public void init () // Obtain the size
of the applet's drawing area. width
getSize ().width height getSize
().height // Create image buffer.
imBuffer createImage (width, height)
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Implement Applet (6-6)
// Attach a mouse motion listener to the applet.
That listener // listens for mouse drag events.
addMouseMotionListener (new
MouseMotionAdapter ()
public void mouseDragged
(MouseEvent e)
if
(inDrag)
//
Calculate draggable checker's new
// origin (the
upper-left corner of
// the checker rectangle). int
tmpox e.getX () - relx
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Checklist before submission

• Project Due 4/18/2007 midnight
• Submit a zip file(david_smith_px.zip) by
  blackboard system, which includes
• Readme.txt
• Report.doc
• Source code
• Compiled classes