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Java Lec02

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History

  • Java
  • Based on C and C++
  • Developed in 1991 for intelligent consumer electronic devices
  • Green Project (According to Gosling, “the goal was … to build a system that would let us do a large, distributed, heterogeneous network of consumer electronic devices all talking to each other.” )
  • James Gosling Modified C++
  • Named Oak then Java.
  • Platform independent
  • Interpreted Language
  • Intermediate Code (Byte Code)
  • Reliable
  • Multiple inheritance and Operator overloading removed
  • No Pointers because of security reasons
  • Built in network support .
  • Internet exploded in 1993, saved project
    • Reliability
    • Security
    • Platform Independence
  • Java formally announced in 1995
  • Now used to create interactive web applications, distributed enterprise application for consumer devices (pagers, cell phones) and much more..

Why Java???

  • Portable WORA!!!!!!
  • Simple
  • “Pure” Object Oriented Language
  • Support for Distributed and Web Applications
  • Rich Libraries
  • Multithreading , Swing , RMI , NET , SQL …..
  • Automatic Garbage Collection
  • More Robust
  • The Java Virtual Machine becomes the common denominator
  • Bytecodes are common across all platforms
  • JVM hides the complexity of working on a particular platform

Java Platform

  • Micro Edition (Java Technology Enabled Devices)
  • Standard Edition (Java Technology Enabled Desktop) & (Workgroup Server)
  • Enterprise Edition (Workgroup Server) & (High End Server)

JAVA vs Other Programming Languages

  • Similar to C/C++ in syntax
  • In-fact Java is C++ minus
  • operator overloading
  • direct pointer manipulation or pointer arithmetic
  • multiple inheritance
  • Destructors (Garbage Collector– handles memory automatically)
  • No Templates
  • Header/make files
  • Lots of other things which make Java more attractive.

Object-Oriented

  • Fundamentally based on OOP
  • All functions belong to classes or objects. No global variables or functions exist
  • All classes by default inherit from a common ancestor known as “Object”
  • “Almost” all data types are objects
  • OOP will be covered in a little more detail later.

Distributed / Network Oriented

  • Java grew up in the days of the Internet
  • Inherently network friendly
  • Original release of Java came with Networking libraries
  • Newer releases contain even more for handling distributed applications
  • RMI, Transactions

Support for Web and Enterprise Applications

  • Given below are some of the Java technologies that can be used for web and enterprise application development
  • Servlets
  • JSP
  • Applets
  • JDBC
  • RMI
  • EJBs
  • JSF

Robust / Secure / Safe

  • Designed with the intention of being secure
  • No pointer arithmetic or memory management!
  • The JVM “verifier”
  • Checks integrity of byte-codes
  • Dynamic runtime checking for pointer and array access
  • No buffer overflow bugs!
  • SecurityManager to check which operations a piece of code is allowed to do
  • “Sandbox” operation for applets and other untrusted code
  • Limited set of operations or resources made available
  • Contrast to ActiveX
  • Rich Set of Libraries
  • Multithreading
    • Swing
    • Regular Expression
    • NET
    • SQL
    • Util
    • Serialization

Java Programmer Efficiency

  • Faster Development
  • More programmer friendly
  • Less error prone
  • OOP
  • Easier to manage large development projects
  • Robust memory system
  • No pointer arithmetic and manual memory management. Garbage collector!
  • Libraries
  • Re-use of code

Disadvantages

  • Java performance IS slower than C/C++
  • Tradeoff between development time vs. run time
  • Additional checks in Java which make is secure and robust and network aware etc, all have a small cost.
  • JIT compilation and HotSpot
  • Dynamic compilation of bytecode to native code at runtime to improve performance
  • HotSpot optimizes code on the fly based on dynamic execution patterns
  • Can sometimes be even faster than compiled C code!
  • Increasing processing speeds helps in overcoming this short fall

Some Sample Java Based Applications

  • Hubble Space Telescope Monitoring
  • Mars Pathfinder Mission Simulator
  • IntelliBrain™ -Bot
  • Star Office 5.2
  • Web Based
    School Management System

Writing Basic Java Program

  • For the start following software will do the job
  • You need to have the latest version of JDK. (J2SE ) You can download it for free from
  • http://java.sun.com/j2se/
  • A little older versions such JDK
  • Notepad / or Other Modern Intelligent Text Editor
  • And you should set your path variable.

 

Example

/* The HelloWorldApp class implements an application that simply displays “Hello World!” to the standard output. */

public class HelloWorldApp {

public static void main(String[] args) {

//Display the string. No global main

System.out.println(“Hello World!”);   }

}

Compiling and Running the program

  • Save this file in some directory and compile it using
  • javac HelloWorldApp.java
  • Run the compiled file by using the command
  • java HelloWorldApp

Java Program Development and Execution Steps

Java Systems

Consist of environment, language, Java Applications Programming Interface (API)

Java programs have five phases

  1. Edit

.java extension

  1. Compile

javac command:  javac MyProgram.java

Creates .class file containing bytecodes with similar name

  1. Loading

Class loader transfers .class file into memory

Classes loaded and executed by interpreter with java command

To load, appletviewer Welcome.html

OR

java MyProgram

  1. Verify

Bytecode verifier makes sure bytecodes are valid and do not violate security

  1. Execute

Computer interprets program one bytecode at a time

Performs actions specified in program.

 

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