Comp2513 Java and Applets Daniel L. Silver, Ph.D

Comp2513Comp2513

Java and AppletsJava and Applets

Daniel L. Silver, Ph.D.Daniel L. Silver, Ph.D.

2001 Daniel L. Silver 2

ObjectivesObjectives

To introduce the Java programming To introduce the Java programming language and its fit with the Weblanguage and its fit with the Web

To discuss the use of Applets as a part of To discuss the use of Applets as a part of the E-Commerce infrastructurethe E-Commerce infrastructure

References: Chapter 3References: Chapter 3


OutlineOutline Java an Object Oriented Programming Java an Object Oriented Programming

languagelanguage Why Java and the Web?Why Java and the Web? Java AppletsJava Applets


Programming LanguagesProgramming Languages A programming language adheres to a A programming language adheres to a

specified syntax that is accepted by either specified syntax that is accepted by either an interpreter or a compiler.an interpreter or a compiler.

What is the difference between an What is the difference between an interpreter and a compiler?interpreter and a compiler?

Basic code

Interpreter Compiler

C code

Operating System

Computer Hardware

*.exe


Function-Oriented ProgrammingFunction-Oriented Programming

Traditional programming languages are known as Traditional programming languages are known as function-oriented. Why?function-oriented. Why?– Consider the movement of data from function to Consider the movement of data from function to

functionfunction– Data and processing are considered separatelyData and processing are considered separately

What are some examples of function-oriented What are some examples of function-oriented languages?languages?– COBOLCOBOL– BasicBasic– FortranFortran– CC

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


Object-Oriented ProgrammngObject-Oriented Programmng

Combines data and processes together into Combines data and processes together into objects objects

An object is an entity that can contain data An object is an entity that can contain data (attributes, properties) and can manipulate (attributes, properties) and can manipulate data using functions (methods)data using functions (methods)

An object has state and behaviourAn object has state and behaviour What are some examples of OO lang.?What are some examples of OO lang.? A simple example … Hello_app.javaA simple example … Hello_app.java

Attributes

Methods


Java: An OOP LanguageJava: An OOP Language

Java is a relatively new language Java is a relatively new language ““Green”, 1991 Sun Microsystems – dev. for Green”, 1991 Sun Microsystems – dev. for

use in consumer devices such as intelligent use in consumer devices such as intelligent TV controllers TV controllers – Object Oriented but simpler than C++Object Oriented but simpler than C++– Architecture neutralArchitecture neutral– Real-time remote applicationsReal-time remote applications– Portable, Reliable and SecurePortable, Reliable and Secure


Java and the Java and the Java Virtual MachineJava Virtual Machine

program.class

Java Virtual Machine Compiler

program.java

Operating System

Computer Hardware


Java Translation and ExecutionJava Translation and Execution

Java sourcecode

Java compiler

Java (JVM) Interpreter

Bytecode compiler

Java bytecode

Machine code

hello_app.java

hello_app.class

hello_app.exe

Efficientbecause of bytecode

javac

java


Java and the WebJava and the Web

In 1995 the first HotJava browser was In 1995 the first HotJava browser was demoed at SunWorld exhibitiondemoed at SunWorld exhibition

It could download programs called It could download programs called applets applets from a the web and run them locallyfrom a the web and run them locally

Provided animation and interactionProvided animation and interaction ““Write once, run anywhere” Write once, run anywhere” By 1996 both Netscape and MS supported By 1996 both Netscape and MS supported

Java within their browsersJava within their browsers



Java has been designed for the WebJava has been designed for the Web It has two attributes that make it suitable:It has two attributes that make it suitable:

– Security:Security: An assortment of security features An assortment of security features that guarantee that no evil applets can be that guarantee that no evil applets can be written and assist in the writing of good codewritten and assist in the writing of good code

– Portability:Portability: Applications and Applets can Applications and Applets can run on Windows, Unix, Linux, Mac, IBM mid-run on Windows, Unix, Linux, Mac, IBM mid-range and mainframerange and mainframe


Java Applet SecurityJava Applet Security

Programs that come from over the network can be Programs that come from over the network can be malicious (destroy data on your PC)malicious (destroy data on your PC)

Java was designed to prevent malicious behaviourJava was designed to prevent malicious behaviour Two primary safety features:Two primary safety features:

– SignaturesSignatures – an applet can be signed and a browser can – an applet can be signed and a browser can be set up to accept only trusted applet authorsbe set up to accept only trusted applet authors

– Secuirty PriviledgesSecuirty Priviledges – by default an applet runs in the – by default an applet runs in the “sandbox” where I/O is limited to the keyboard/mouse “sandbox” where I/O is limited to the keyboard/mouse and the display, trusted applets can be give higher level and the display, trusted applets can be give higher level priveledges (e.g. disk I/O )priveledges (e.g. disk I/O )



Reasons why Java has become the fastest Reasons why Java has become the fastest growing programming language of all time:growing programming language of all time:– It is an object-oriented languageIt is an object-oriented language– Contains a vast library of software (object Contains a vast library of software (object

classes and methods) classes and methods) » Java Development Kit (J2SE SDK)Java Development Kit (J2SE SDK)

» Objects that have been developed and testedObjects that have been developed and tested

» Imported for use at the beginning of a program Imported for use at the beginning of a program

– A good first language to learn ... Why?A good first language to learn ... Why?


Java Applications Java Applications vsvs Applets Applets

There are two classes of Java programs:There are two classes of Java programs: Applications - such as Hello_app.javaApplications - such as Hello_app.java

– I/O is by default with console and character orientedI/O is by default with console and character oriented– Graphical I/O is an option that is commonly takenGraphical I/O is an option that is commonly taken

Applets – as we shall seeApplets – as we shall see– Run by a Web browser using an imbedded Java Run by a Web browser using an imbedded Java

interpreterinterpreter– Graphical I/O – characters, diagramsGraphical I/O – characters, diagrams


Java Translation and ExecutionJava Translation and Execution

Java sourcecode

Java compiler

Java appletbytecode

Internet

Internet

Java (JVM)Interpreter

Web Browser

Client

Server

abc.java

abc.class HTTP ServerApp Server


Java AppletsJava Applets

A Simple Example:A Simple Example: HelloApplet.javaHelloApplet.java / / HelloApplet.htmlHelloApplet.html … … Nothing is passed to the applet from the HTMLNothing is passed to the applet from the HTML Anatomy of an Applet – fundamental methods:Anatomy of an Applet – fundamental methods:

– init() – invoked once when applet is first loadedinit() – invoked once when applet is first loaded

– start() – invoked each time applet becomes visiblestart() – invoked each time applet becomes visible

– stop() – invoked each time applet becomes invisiblestop() – invoked each time applet becomes invisible

– paint() – display of text and graphicspaint() – display of text and graphics

– destroy() – invoked once when applet is exiteddestroy() – invoked once when applet is exited



Parameter Passing Example:Parameter Passing Example: FirstApplet.java / FirstApplet.html …FirstApplet.java / FirstApplet.html … <PARAM> tag is used to pass parameters from <PARAM> tag is used to pass parameters from

HTML to Java program at run time HTML to Java program at run time <PARAM> has two attributes:<PARAM> has two attributes:

– NAME - name of parameter being passedNAME - name of parameter being passed

– VALUE – value of parameter being passedVALUE – value of parameter being passed

- e.g.:e.g.:<PARAM NAME="message" VALUE="Message from HTML File"><PARAM NAME="message" VALUE="Message from HTML File">



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



Threads and Event Handling Example:Threads and Event Handling Example: AnimationApplet.java / AnimationApplet.java /

AnimationApplet.html …AnimationApplet.html … Thread – a portion of a Java program that Thread – a portion of a Java program that

executes independently, e.g.:executes independently, e.g.:– Thread 1 - allows animation to occur on the Thread 1 - allows animation to occur on the

browser windowbrowser window– Thread 2 – captures input from the keyboard Thread 2 – captures input from the keyboard



Threads and Event Handling Example:Threads and Event Handling Example: Event Handling is used within programs to Event Handling is used within programs to

associate events such as mouse clicks to associate events such as mouse clicks to revelent portions of coderevelent portions of code

In AnimationApplet.java a mouseDown() In AnimationApplet.java a mouseDown() event is used to start and stop the scrolling event is used to start and stop the scrolling messagemessage



A more advanced example – link.htmlA more advanced example – link.html– Provides sources of further information on Java Provides sources of further information on Java

and Appletsand Applets Notice how in link.java:Notice how in link.java:

– <param> is used in this example<param> is used in this example– showDocument(theURL,targetFrame) is used to showDocument(theURL,targetFrame) is used to

link to a URLlink to a URL

THE ENDTHE END

[email protected]@acadiau.ca

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Comp2513 Java and Applets Daniel L. Silver, Ph.D