Comp2513
Daniel L. Silver, Ph.D.

 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
2001 Daniel L. Silver 2

 Java an Object Oriented Programming language
 Why Java and the Web?
 Java Applets
2001 Daniel L. Silver 3

 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?
Basic code C code
Interpreter Compiler
Operating System
*.exe
2001 4

 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?
•
Prone to misuse of data and process
– COBOL
– Basic elements
• Not supportive of encapsulation,
– Fortran
– C cohesion and loose coupling
2001 Daniel L. Silver 5

 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)
Attributes
 An object has state and behaviour
Methods  What are some examples of OO lang.?

A simple example … Hello_app.java
2001 Daniel L. Silver 6

 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
2001 Daniel L. Silver 7

2001 program.class
program.java
Java Virtual Machine
Operating System
Compiler
Computer Hardware
Daniel L. Silver 8

Java source code
Java compiler javac hello_app.java
Java bytecode hello_app.class
Efficient because of bytecode
Java (JVM)
Interpreter java
2001
Bytecode compiler
Daniel L. Silver
Machine code hello_app.exe
9

 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
2001 Daniel L. Silver 10

 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 midrange and mainframe
2001 Daniel L. Silver 11

 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 )
2001 Daniel L. Silver 12

 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?
2001 Daniel L. Silver 13

vs
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
2001 Daniel L. Silver 14

Java source code abc.java
Java compiler abc.class
Java applet bytecode
HTTP Server
App Server t
Interne
2001
Server
Daniel L. Silver
Web Browser
Java (JVM)
Interpreter
Client
15

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
2001 Daniel L. Silver 16

Parameter Passing Example:
 FirstApplet.java
/ FirstApplet.html
…
 <PARAM> tag is used to pass parameters from
HTML to Java program at run time
 <PARAM> has two attributes:
– NAME - name of parameter being passed
– VALUE – value of parameter being passed
- e.g.:
<PARAM NAME="message" VALUE="Message from HTML File">
2001 Daniel L. Silver 17

 The Graphics object class allows you to do many things:
– setColor()
– fillRect()
– drawLine()
– drawRect()
– drawOval()
– drawImage()
2001 Daniel L. Silver 18

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
2001 Daniel L. Silver 19

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
2001 Daniel L. Silver 20

 A more advanced example – link.html
– Provides sources of further information on Java and Applets
 Notice how in link.java
:
– <param> is used in this example
– showDocument(theURL,targetFrame) is used to link to a URL
2001 Daniel L. Silver 21

danny.silver@acadiau.ca