Introduction to Java

1
Introduction to Java

• April 10, 2008
• David Goldschmidt, Ph.D.
• goldschd_at_strose.edu

2
Java Applets

• When an applet is loaded, the Web browser invokes
  the applets no-argument constructor
• Web browser uses init(), start(), stop(),and
  destroy() methods to control the applet
• By default, these methods do nothing
• Override these methods in your applet such
  thatthe browser calls your code properly

3
Java Applets

• Web browser calls init() when applet is first
  loaded (and again if applet is reloaded)
• Override init() to
• Set up UI components
• Create new threads
• Load images
• Read parameter values
  from ltappletgt tag

4
Java Applets

• After init() and on returning to a Web page, the
  Web browser calls the start() method
• Override start() to
• Reset UI components
• Restart animations
• Restart threads

5
Java Applets

• When the user leaves the containing Web page, the
  Web browser calls the stop() method
• Override stop() to
• Stop animations
• Suspend threads

6
Java Applets

• When the user closes the containing Web page, the
  Web browser calls the destroy() method
• Override destroy() to
• Free allocatedresources
• Discard threads

7
Applications vs. Applets

• JFrame and JApplet are both subclasses of
  Container
• All user interface components, layout
  managers,and event-handling features are the
  same
• Applets require HTML for deployment
• Both applications and applets require a JVM
• (via JRE or Web browser plug-in)

8
Applications vs. Applets

• Applications are invoked from astatic main()
  method by the JVM
• Applets are run by Web browser
• To provide a graphical environment
• Applications require a frame (i.e. JFrame)
• Applets require a Web browser

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Applications vs. Applets

• Applets have security restrictions
• An applet is not allowed to read from orwrite to
  the file system of the Web browsers computer
• An applet is not allowed to run any programson
  the Web browsers computer
• An applet is not allowed to establish connections
  between the users computer and another computer
• Except with the server on which the applet is
  stored

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Applications vs. Applets

• Converting an applet into an application is
  always possible (and usually mechanical)
• Converting an application into an applet is
  sometimes possible, depending on whether security
  restrictions on applets are not violated

11
Tic Tac Toe
12
Tic Tac Toe
13
Bouncing Ball

• Bouncing ball applet displays a ball bouncing
  within a panel
• Two buttonssuspend andresume movement
• Scroll bar controlsspeed
• Run as either anapplet or application

14
Bouncing Ball
15
Overcoming Applet Restrictions

• Security restrictions disallow applets from
  accessing local or network files
• Applets can load image, audio, video,and other
  local or network files
• Use the java.net.URL class to identifylocal
  files or files available via the Internet

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Deploying Java Applets

• A Java Archive ( JAR) file groups all project
  files into a single compressed file for
  deployment
• Includes Java class files, images, audio files,
  etc.
• Enables efficient deployment ofboth Java applets
  and applications
• JAR files are based on the ZIP file format
• Can usually open JAR files using WinZip

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Deploying Java Applets

• Use jar command from the Java SDK to create a
  Java archive file
• jar -cvf TicTacToe.jar TicTacToe.class
  TicTacToeCell.class
• Command-line arguments very similar to tar
• Use -c to create a new JAR file
• Use -v to specify verbose mode
• Use -f to specify the name of the JAR file

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Deploying Java Applets

• Use the following HTML to deploy the JAR file
• ltapplet
• code"TicTacToe.class"
• archive"TicTacToe.jar"
• width"350"
• height"200"gt
• lt/appletgt