got

1

• got ?

Research Project April 1998 Hang Xia, Mark
Wang, Richard S. Chang Updated R Norman, August
1999
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• Why is Java so Hot?
• because of the NETWORK!!
• Why Java?
• Write Once, Use Anywhere

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Agenda

• Java Virtual Machine
• Java App/Servlet JavaScript
• Java Language

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• Java
• Virtual
• Machine

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Java Virtual Machine

• What is the JVM?
• A Java Interpreter
• Software, not hardware
• Converts bytecode into machine code
• Why JVM is important?
• Platform-Independent

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Java Development andJava Runtime Environment
(JRE)

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Java Development Kit (JDK)

• A development environment for writing
• Java Applets Servlets
• Java Applications
• JDK contains
• Java Core Classes
• Java Source Files
• Java Tools (compiler, Interpreter, appletviewer,
  debugger, etc.)
• Java Documentation and Demos

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• Java Applet
• Java Servlet
• JavaScript

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What is a Java Applet?

• A Java program that can be included in an HTML
  page.
• An applet tag is used in the HTML file to
  identify the source of the applet code, which
  resides on the server.
• Example applet tag in HTML file
• ltapplet codeMyApplet.class width100
  height80gt
• lt/appletgt

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How a Java Applet works

• When we use a Java-compatible browser to view a
  Web page which contains a Java applet, the
  applets code is transferred to our system and
  executed by the browser.
• Java-Compatible Browsers HotJava, Netscape
  Navigator, Internet Explorer, etc.

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Why a Java Applet?

• ADVANTAGES
• Less load on the server
• More user interaction (Applet Vs. CGI)
• Arbitrary graphics
• Arbitrary data types

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Java Applets Vs. Java Applications

• Java Application
• Standalone Java programs that can be run by
  using just the Java interpreter.
• Java Applet
• Run and displayed inside a Web page with
• Java-enabled Web browser.

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Embedding Applet in HTML

• ltAPPLET
• CODEBASE codebaseURL
• ARCHIVE archiveList
• CODE appletFile ...or... OBJECT
  serializedApplet
• ALT alternateText
• NAME appletInstanceName
• WIDTH pixels HEIGHT pixels
• ALIGN alignment
• VSPACE pixels HSPACE pixels gt
• ltPARAM NAME appletAttribute1 VALUE
  valuegt
• ltPARAM NAME appletAttribute2 VALUE
  valuegt
• . . .
• alternateHTML
• lt/APPLETgt
  ( Applet Tag )

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What is JavaScript?

• JavaScript is a cross-platform, object-based
  scripting language for client and server
  applications.
• Untyped language
• Purely interpreted language

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Is JavaScript Simplified Java?
NO
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JavaScript Vs. Java

• JavaScript -- Interpreted (not compiled) by
  client. Code integrated with and embedded in
  HTML.
• Java -- Compiled bytecodes downloaded from
  server, executed on client. Applets distinct
  from HTML (accessed from HTML pages).

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Embedding JavaScript in HTML

• Explicit Way
• ltSCRIPTgt
• JavaScript
  statements
• lt/SCRIPTgt
• Implicit Way
• ltSCRIPT
  SRCcommon.jsgt
• lt/SCRIPTgt

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• Java
• Language

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Java Language

• Syntactically similar to C
• Semantically different
• Strong and statically typed
• Single inheritance plus an interface mechanism
• Run-time type information
• Automatic storage management
• No pointers
• Garbage collection
• Security/safety

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Advantages of Java

• Combine many programming mechanisms
• Object oriented language
• Strongly and statically typed
• Automatic storage management garbage collection
• Run-time type information
• Safety
• Faster build cycle (prototyping)
• Dynamic linking

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Advantages of Java (cont.)

• Platform independence
• The ability of write code once and use it
  anywhere
• The ultimate in portability
• Compiled code can move around a network
• Java is Open
• Documentation and source code available
• Not tied down to a single vendor (i.e. Sun or
  Microsoft)

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Disadvantages of Java

• Implementations have traditionally been
  interpreted
• Execute slower than compiled languages
• Typically in the order of 10x slower than native
  code
• Write Once, Run Anywhere Implementation
  challenges
• Is Microsoft playing????

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Build Cycle
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Build Cycle (cont.)
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JIT Compilation

• Just-in-time compilation
• Convert Java bytecodes to native code as they are
  executed
• Total execution time compilation time run time

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• Questi ns ?