Java Introduction

 

Introduction to Java Programming

·        Object-oriented programming

·        Multi-threaded Programming

·        Developed by Sun Microsystems

 

 

Java authorà James Gosling, found java in 1991.

Java first names "Oak" but the copyright problem to Oak was renamed as "java"in 1995.

 

Features of Java:

 

*Simple                              *Portable

*Object oriented              *Interpreted

*Network-savvy               *High performance

*Robust                              *Multithreaded

*Secure                              *Dynamic

*Architecture Neutral

 

Difference between JDK, JRE, and JVM

JVM

JVM (Java Virtual Machine) is an abstract machine. It is called a virtual machine because it doesn't physically exist. It is a specification that provides a runtime environment in which Java bytecode can be executed. It can also run those programs which are written in other languages and compiled to Java bytecode.

The JVM performs the following main tasks:

• Loads code
• Verifies code
• Executes code
• Provides runtime environment

JRE

JRE is an acronym for Java Runtime Environment. It is also written as Java RTE. The Java Runtime Environment is a set of software tools which are used for developing Java applications. It is used to provide the runtime environment. It is the implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime.

The implementation of JVM is also actively released by other companies besides Sun Micro Systems.

 

JDK

JDK is an acronym for Java Development Kit. The Java Development Kit (JDK) is a software development environment which is used to develop Java applications and applet

. It physically exists. It contains JRE + development tools.

JDK is an implementation of any one of the below given Java Platforms released by Oracle Corporation:

• Standard Edition Java Platform
• Enterprise Edition Java Platform
• Micro Edition Java Platform

The JDK contains a private Java Virtual Machine (JVM) and a few other resources such as an interpreter/loader (java), a compiler (javac), an archiver (jar), a documentation generator (Javadoc), etc. to complete the development of a Java Application.

 

Creating a java  Application

/*

This is a simple java program. Call this file Hello World. java.

*/

class HelloWorld{

 //Your program begins with a call to main().

public static void main (String args[]){

        System.out.println("Hello World!");

}

}

Output

To compile java Program

>javac filename.java

After compilation there is no error filename.class file will be created.

To Run java Program

>java filename.java

Example

>javac HelloWorld.java

>java HelloWorld

Comment

line hiding

//-Single Line Comment - To hide a single line single line comment used

/*......*/-Multi Line Comment - To hide a multiline multiline comment used

 

 

Character escapes codes,

              Escape                            Meaning

                 \n                                   New line

                \t                                      Tab

              \b                                        Backspace

Variable 

Value Storage

Types of Variables

There are three types of variables in Java:

• local variable
• instance variable
• static variable

 

1) Local Variable

A variable declared inside the body of the method is called local variable. You can use this variable only within that method and the other methods in the class aren't even aware that the variable exists.

A local variable cannot be defined with "static" keyword.

2) Instance Variable

A variable declared inside the class but outside the body of the method, is called an instance variable. It is not declared as static.

It is called an instance variable because its value is instance-specific and is not shared among instances.

3) Static variable

A variable that is declared as static is called a static variable. It cannot be local. You can create a single copy of the static variable and share it among all the instances of the class. Memory allocation for static variables happens only once when the class is loaded in the memory.