CS4273: Distributed System Technologies and Programming I

1
CS4273 Distributed System Technologies and
Programming I

• Lecture 3 Java Applets and Animations

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Java Applet

• Applet is a special kind of Java application
  programs that are embedded into web pages. It
  must run in a web-browser (or appletviewer).
• It is downloaded from the network, and executed
  locally. It is the idea of develop once and run
  anywhere.
• Applet tag in HTML
• ltAPPLET
• CODEBASE DirOrURL
• CODE Local.class
• NAME ThisAppletName
• WIDTHpixels HEIGHTpixels
• ALIGN AlignmentValue //LEFT, RIGHT, TOP,
  
• VSPACEpixels HSPACEpixels
• ALT AlternativeTextIfAppletNotWorking gt
• ltPARAM NAME ParameterName VALUE
  ParameterValuegt
• ...
• lt/APPLETgt

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Passing Parameters to Applets

• Applets get parameters from ltPARAMgt fields
• ltAPPLET CODE"JavaGreeting.class" WIDTH100
  HEIGHT50gt
• ltPARAM NAMEGreeting VALUEHello, Good
  Morninggt
• lt/APPLETgt
• Code of getting the parameter value in the
  applet
• String GreetingWords getParameter("Greeting")
  

4
Applet Methods

• Applets do not have a main routine. Applet class
  has many methods. You need to override some of
  them to let applet perform your task. The Applet
  methods are listed below in the order they are
  called
• init() called only once when the applet is
  loaded. It initializes variables and initial
  screen display.
• start() called after init() and every time
  returned to the page containing the applet after
  gone off. It can be called repeatedly.
• paint(Graphics g) it is also called by
  repaint().
• stop() called when moving off the page. When
  coming back, start() method is called. It is used
  to stop time-consuming activities (animation
  threads) when not seen.
• destroy() called when the browser shuts down. It
  deletes the applet and recalls all resources.

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Example of an Applet

• class BtnAdapter implements ActionListener
  
• private int id
• BtnAdapter( int buttonID)
• id buttonID
• public void actionPerformed (ActionEvent
  e)
• switch (id)
• case 0
  musicOnce.play()
• break
• case 1
  musicLoop.loop()
• break
• case 2
  musicLoop.stop()

• public class music extends JApplet
• AudioClip musicOnce, musicLoop
• public void init()
• add (playBtn new Button("Play"))
• playBtn.addActionListener(new BtnAdapter (0))
  add (loopBtn new Button("Loop"))
• add (stopBtn new Button("Stop"))
• stopBtn.addActionListener( new BtnAdapter (2))
• musicOnce getAudioClip(getDocumentBase(),
  TAM.au")
• musicLoop getAudioClip(getDocumentBase(),
  "deng0.au")
• public void start()
• musicLoop.loop()
• public void stop()
• musicLoop.stop()

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paint, repaint paintComponent

• Whenever you need to update the display, call
  repaint() (you cannot call paint() directly).
  repaint() invokes paint().
• In paint(), you usually start with super.paint(),
  which performs some preparation work for you,
  e.g., clear the display area. Without calling
  super.paint(), the paint() will paint on top of
  the existing drawings.
• JComponent has method paintComponent(). For
  painting a sub-region of applets display area
  (i.e., Jcomponent), you need to override
  paintComponent(), instead of paint(). paint()
  invokes paintComponent().

7
Applets and Applications

• JApplet is a subclass of JPanel.
• Create a component and add it to an applet in the
  same way as adding it to a container, e.g.,
  add(new button(start)), add(new Canvas()), etc.

8
Play Audio Files in Applets

• When playing music, you need to get an AudioClip
  object and then play it. Get an audio clip by
• AudioClip getAudioClip(URL url, String name)
• Three methods on AudioClip objects
• play() it plays the audio file once and stop.
• loop() it plays the audio file repeatedly until
  the stop method is called.
• stop() it stops a loop play of an audio clip.

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Example of Playing AudioClips

• class BtnAdapter implements ActionListener
  
• private int id
• BtnAdapter( int buttonID)
• id buttonID
• public void actionPerformed (ActionEvent
  e)
• switch (id)
• case 0
  musicOnce.play()
• break
• case 1
  musicLoop.loop()
• break
• case 2
  musicLoop.stop()

• public class audioPlayer extends JApplet
• AudioClip musicOnce, musicLoop
• public void init()
• add (playBtn new Button("Play"))
• playBtn.addActionListener(new BtnAdapter (0))
  add (loopBtn new Button("Loop"))
• add (stopBtn new Button("Stop"))
• stopBtn.addActionListener( new BtnAdapter (2))
• musicOnce getAudioClip(getDocumentBase(),
  sun.au")
• musicLoop getAudioClip(getDocumentBase(),
  "deng.au")
• public void start()
• musicLoop.loop()
• public void stop()
• musicLoop.stop()

10
Play Video in Applets

• Java Media Framework (JMF) API is used to play
  and edit media files. You need to
• download JMF package to IE to enable JMF
  functions http//java.sun.com/javase/technologies/
  desktop/media/jmf/2.1.1/download.html and,
• compile programs with JMF library.
• The main JMF API includes
• A Manager class contains methods for playing and
  manipulating media clips.
• Create a player object out of URL of a video clip
  by using Manager method
• Player player Manager.createRealizedPlayer(me
  diaURL)
• Get a video display panel and a control panel,
  and add them to applet
• videoPanel player.getVisualComponent()
• controlPanel player.getControlPanelComponent()
• add(center, videoPanel)
• add(south, controlPanel)
• Control methods on the player object
• player.start()
• player.stop()
• player.close()

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Example of Playing Video in Applets

• public class videoPlayer extends JApplet
• Player player
• public void init()
• URL mediaURL new
  URL(getDocumentBase(),"bailey.mpg")
• player Manager.createRealizedPlay
  er(mediaURL)
• Component video
  player.getVisualComponent()
• Component controls
  player.getControlPanelComponent()
• add("Center", video)
• add("South", controls)
• public void start()
• if (player ! null)
  player.start()
• public void stop()
• if (player ! null)
  player.stop()

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Display Images

• get an Image by
• Image getImage(URL url, String name)
• paint the image by
• drawImage(image, x, y, observer)
• An example
• public class ImageTest extends Applet
• Image theImage null
• public void init()
• theImage getImage(getDocumentBas
  e(), "pg9.gif")
• public void paint(Graphics g)
• g.drawImage(theImage, 0, 0,
  this)

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Eample of Animation using Thread

• public class animation extends JApplet implements
  Runnable
• Thread runner
• Vector frames new Vector()
• int cur_frame, frame_delay 400
• public void init()
• URL base getDocumentBase()
• for (int i 1 i lt 8 i)
• Image img
  getImage(base,
• "image/bunny" i ".gif")
• frames.addElement(img)
• public void start()
• if (runner null)
• runner new
  Thread(this)
• runner.start()
• public void stop()
• runner null

• public void run()
• cur_frame 1
• while(runner ! null)
• Thread.sleep(frame_delay)
  
• repaint()
• if(cur_frame gt
  frames.size())
• cur_frame 1
• else
• cur_frame
• public void paint(Graphics g)
• super.paint(g)
• Image img
• (Image)frames.elementAt(cur_frame-1)
• g.drawImage(img, cur_frame30, 0,
  this)

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Steps of a Simple Animation

• Animation using Timer class
• Create a Timer object
• new Timer(delay, new TimerListener())
• The timer generates an action event in interval
  of delay and TimerListener() is an actionListener
  class that handles this action event.
• Methods for Timer control
• timer.setDelay(delay)
• timer.start()
• timer.stop()

• Animation using Thread
• In init(), load the images into frames (data
  structure of a vector).
• In start(), start a thread to do the animation.
• In run() of the thread, paint the cur-frame,
  pause a while, and move forward the pointer of
  frames.
• In paint(), paint the image of the cur-frame.

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Example of Animation using Timer

• public void paint(Graphics g)
• .......
• x dx
• y dy
• g.setColor(Color.red)
• g.fillOval(x-radius, y-radius,
  radius2, radius2)
• class TimerListener implements ActionListener
• public void actionPerformed(Action
  Event e)
• repaint()
• class ButtonAdapter implements ActionListener
• public void actionPerformed
  (ActionEvent e)
• if(e.getSource()
  btSuspend)
• timer.stop()
• if(e.getSource()
  btResume)
• timer.start()

• public class bounceBall extends JApplet
• int delay 100, x 0, y 20, radius
  10, dx 10, dy 10
• public Timer timer new Timer(delay, new
  TimerListener())
• public void init()
• panel.add(btSuspend new
  JButton("Suspend"))
• btSuspend.addActionListener(new
  ButtonAdapter())
• panel.add(btResume new
  JButton("Resume"))
• btResume.addActionListener(new
  ButtonAdapter())
• JScrollBar scrollBar new
  JScrollBar()
• scrollBar.setOrientation(JScrollBa
  r.HORIZONTAL)
• scrollBar.addAdjustmentListener(ne
  w ScrollAdapter())
• add(scrollBar, BorderLayout.NORTH)
  
• add(displayArea,
  BorderLayout.CENTER)
• add(panel, BorderLayout.SOUTH)
• timer.start()
• public void start()

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Media Tracker

• MediaTracker class provides methods to check
  whether the loading of an image (a group of
  images) is complete or not, and get status of
  image loading. It is particularly useful in
  managing a group of images.
• Construct a MediaTracker object by
• MediaTracker myTracker new MediaTracker(this)
• Useful methods of MediaTracker class
• addImage(img, id), add the img into the Tracker
• isErrorAny(), if loading process has an error
• checkID (id), check if the image is loaded
• checkAll (), if all images are loaded
• waitForID (id),
• waitForAll (), block the current executing thread
  until all images are loaded.

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Example of using MediaTracker

• public class ImageApplet extends JApplet
• Image myImg null
• MediaTracker myTracker null
• public void init()
• myTracker new MediaTracker(this)
• myImg getImage(getDocumentBase(), me.gif)
• myTracker.addImage(myImg, 0)
• public void paint(Graphics g)
• if (myTracker.isErrorAny())
• g.drawString(errors in loading, 10, 10)
• return
• else if (myTracker.checkAll(true))
• g.drawImage(myImg, 0, 0, this)
• else
• g.drawString(image loading, 10, 10)
• repaint(100) // recursive call paint every
  100ms

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Applet Network Access

• Security of an applet
• applets cannot run any local executable program.
• applets cannot read or write to the local
  computers file system.
• applets cannot communicate with any host other
  than the server from which they are downloaded.
• Getting data from originating server
• applets get data from their originating servers
• URL getDocumentBase() // URL of this web page
• URL getCodeBase() // URL of this of this applet
• Examples
• cat getImage (getDocumentBase(),
  images/cat.gif)

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Message Passing between Applets

• When a web page contains several applets, it is
  often required that the applets communicate with
  each other. An applet can send messages to
  another applet on the same page (communicating
  applets must originate from the same server).
• An applet sends a message to another via invoking
  a method of the other applet. It is inter-applet
  method invocation.
• Applets identify each other by using their
• names (assigned in the HTML file).
• The message passing is done inside the
• browser (at the client side) without the
• server involved.

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HTML File Containing two Applets with Names

• lthtmlgt
• ltbodygt
• ltAPPLET CODE"sender.class" WIDTH450 HEIGHT300
  NAMEBob"gt
• ltPARAM NAMERECEIVER VALUEAlice"gt
• lt/APPLETgt
• ltAPPLET CODE"receiver.class" WIDTH450
  HEIGHT300 NAMEAlice"gt
• lt/APPLETgt
• lt/bodygt
• lt/htmlgt

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Steps for sending a message to another applet

• 1. Sender gets receivers name by
• recverName getParameter("RECEIVER")
• 2. Sender gets the receiver applet by
• recver getAppletContext().getApplet(recve
  rName)
• 3. Sender invokes the receivers method by
• reply_msg ((receiver) recver).recvMsg(msg)
  
• // recvMsg is a method of the receiver
  (for receiving messages).
• 4. The receivers method can return a value (or
  string) that the sender can get as the reply of
  its message.

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The sender program

• // sender.java
• public class sender extends Applet implements
  ActionListener
• public void init()
• recvName getParameter("RECEIVER")
• //... set the GUI
• public void actionPerformed(ActionEvent event)
• String reply_msg, msg test of applet message
  passing"
• Applet recver null
• //Get the receiver applet
• recver getAppletContext().getApplet(recverName)
  
• // call receiver applets method.
• reply_msg ((receiver)recver).recvMsg(myName,
  msg)
• status.append(Replied msg " reply_msg
  "\n")
• msg reply_msg

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The receiver program

• // receiver.java
• public class receiver extends JApplet implements
  ActionListener
• private JButton button new
  JButton("Clear")
• private JTextArea status new
  JTextArea(5, 60)
• public void init()
• button.addActionListener(this)
  add("North", button)
• add("Center", status)
• add("South", new JLabel("My name
  is " getParameter("NAME")))
• public String recvMsg(String senderName, String
  msg)
• status.append("Received from "
  senderName " " msg "\n")
• i
• return msgi // echo received
  message back
• public void actionPerformed(ActionEvent event)
• status.setText("")
• i 0