Comp2513 Java and Applets

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Comp2513Java and Applets

• Daniel L. Silver, Ph.D.

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Objectives

• To introduce the Java programming language and
  its fit with the Web
• To discuss the use of Applets as a part of the
  E-Commerce infrastructure
• References Chapter 3

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Outline

• Java an Object Oriented Programming language
• Why Java and the Web?
• Java Applets

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Programming Languages

• A programming language adheres to a specified
  syntax that is accepted by either an interpreter
  or a compiler.
• What is the difference between an interpreter and
  a compiler?

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Function-Oriented Programming

• Traditional programming languages are known as
  function-oriented. Why?
• Consider the movement of data from function to
  function
• Data and processing are considered separately
• What are some examples of function-oriented
  languages?
• COBOL
• Basic
• Fortran
• C

• Prone to misuse of data and process
• elements
• Not supportive of encapsulation,
• cohesion and loose coupling

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Object-Oriented Programmng

• Combines data and processes together into objects
  
• An object is an entity that can contain data
  (attributes, properties) and can manipulate data
  using functions (methods)
• An object has state and behaviour
• What are some examples of OO lang.?
• A simple example Hello_app.java

Attributes
Methods
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Java An OOP Language

• Java is a relatively new language
• Green, 1991 Sun Microsystems dev. for use in
  consumer devices such as intelligent TV
  controllers
• Object Oriented but simpler than C
• Architecture neutral
• Real-time remote applications
• Portable, Reliable and Secure

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Java and the Java Virtual Machine
program.java
program.class
Java Virtual Machine
Compiler
Operating System
Computer Hardware
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Java Translation and Execution
Java source code
hello_app.java
Java bytecode
Java compiler
hello_app.class
javac
Bytecode compiler
Efficient because of bytecode
Java (JVM) Interpreter
java
Machine code
hello_app.exe
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Java and the Web

• In 1995 the first HotJava browser was demoed at
  SunWorld exhibition
• It could download programs called applets from a
  the web and run them locally
• Provided animation and interaction
• Write once, run anywhere
• By 1996 both Netscape and MS supported Java
  within their browsers

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Java and the Web

• Java has been designed for the Web
• It has two attributes that make it suitable
• Security An assortment of security features that
  guarantee that no evil applets can be written and
  assist in the writing of good code
• Portability Applications and Applets can run on
  Windows, Unix, Linux, Mac, IBM mid-range and
  mainframe

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Java Applet Security

• Programs that come from over the network can be
  malicious (destroy data on your PC)
• Java was designed to prevent malicious behaviour
• Two primary safety features
• Signatures an applet can be signed and a
  browser can be set up to accept only trusted
  applet authors
• Secuirty Priviledges by default an applet runs
  in the sandbox where I/O is limited to the
  keyboard/mouse and the display, trusted applets
  can be give higher level priveledges (e.g. disk
  I/O )

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Java and the Web

• Reasons why Java has become the fastest growing
  programming language of all time
• It is an object-oriented language
• Contains a vast library of software (object
  classes and methods)
• Java Development Kit (J2SE SDK)
• Objects that have been developed and tested
• Imported for use at the beginning of a program
• A good first language to learn ... Why?

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Java Applications vs Applets

• There are two classes of Java programs
• Applications - such as Hello_app.java
• I/O is by default with console and character
  oriented
• Graphical I/O is an option that is commonly taken
• Applets as we shall see
• Run by a Web browser using an imbedded Java
  interpreter
• Graphical I/O characters, diagrams

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Java Translation and Execution
Java source code
abc.java
Internet
Java compiler
HTTP Server App Server
abc.class
Java applet bytecode
Web Browser
Java (JVM) Interpreter
Server
Client
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Java Applets

• A Simple Example
• HelloApplet.java / HelloApplet.html
• Nothing is passed to the applet from the HTML
• Anatomy of an Applet fundamental methods
• init() invoked once when applet is first loaded
• start() invoked each time applet becomes
  visible
• stop() invoked each time applet becomes
  invisible
• paint() display of text and graphics
• destroy() invoked once when applet is exited

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Java Applets

• Parameter Passing Example
• FirstApplet.java / FirstApplet.html
• ltPARAMgt tag is used to pass parameters from HTML
  to Java program at run time
• ltPARAMgt has two attributes
• NAME - name of parameter being passed
• VALUE value of parameter being passed
• e.g.
• ltPARAM NAME"message" VALUE"Message from HTML
  File"gt

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Java Applets

• The Graphics object class allows you to do many
  things
• setColor()
• fillRect()
• drawLine()
• drawRect()
• drawOval()
• drawImage()

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Java Applets

• Threads and Event Handling Example
• AnimationApplet.java / AnimationApplet.html
• Thread a portion of a Java program that
  executes independently, e.g.
• Thread 1 - allows animation to occur on the
  browser window
• Thread 2 captures input from the keyboard

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Java Applets

• Threads and Event Handling Example
• Event Handling is used within programs to
  associate events such as mouse clicks to revelent
  portions of code
• In AnimationApplet.java a mouseDown() event is
  used to start and stop the scrolling message

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Java Applets

• A more advanced example link.html
• Provides sources of further information on Java
  and Applets
• Notice how in link.java
• ltparamgt is used in this example
• showDocument(theURL,targetFrame) is used to link
  to a URL

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THE ENDdanny.silver_at_acadiau.ca