Java Introduction

Java is a truly object-oriented programming language developed by Sun Microsystems.
Goal was:"Write once, run anywhere, any time, forever"
The language is modeled on C++.

History

• Java was conceived by James Gosling, Patrick Naughton, Chris Warth, ED Frank and Mike Sheridian at Sun Microsystem in 1991.
• Initially Java was named as "Oak", because there was a Oak tree in front of the Sun Micosystems' office. But the name was already registered by some body so that name could not be used.
• So one day the team went to a coffee shop and the name Java was struck to them the name was chosen so.

Features of Java

• Simple
• Secure
• Portable
• Object Oriented
• Robust
• Multithreaded
• Architecture Neutral
• Interpreted
• High Performance
• Distributed
• Dynamic

Object Oriented Concepts

• All tangible things around us are objects.
• Has got a name, a set of attributes (or properties) and behaviors (or functions).

Example :
Class Rose

Objects and Class

• Object oriented programming supports the view that programs are composed of objects that interact with one another.
• Object must have unique identity (name): Car, Bicycle, Red ball.
• Consider a car:
• Color, wheel, engine, car seat, doors are properties(data declaration)
• Move, horn, stop are behaviors(methods)
• Class of an ObjectA class defines a collection of objects of same type.

Example

class Person
{
float height;
int age;
String lastName;
void run(){...}
void eat(){...}
void sleep(){...}
}

• The process of creating an object from a class is called instantiation.

Object-Oriented Principles

Java Working

• The designers of Java used the combination of compilation and interpretation.
• Programs written in java are compiled into machine level language. But it is a machine level language for a computer that does not really exist.
• This is so called Virtual Computer is known as Java Virtual Machine (J.V.M.).
• The machine language for the JVM is called Java Byte Code. This java byte code can be run on any computer.
• But the computer needs an interpreter for the java byte code.
• This java byte code interpreter is different for different types of computer.
• But once the byte code is generated it can be run on any type of machines.
• Writing interpreter is easy and it is small.
• Thus instead of writing compiler for each type of computer, interpreter is written.
• Thus, even if a new type of computer is developed, it is easy to write an interpreter.

How Java works?

• A compiler converts the java program into intermediate language form --Byte code, which is platform independent. This code is for a machine which is not in existence, it is called as Java Virtual Machine (JVM)
• On any machine when you compile java code using javac compiler, byte code equivalent to the source is generated.
• Byte code enables "run-anywhere" capability.
• You invoke java command which will feed the byte code to the machine-dependent interpreter.

Adding main() Method

This is the point where the execution of the program begins.
Object Creation Using 'new'
class Person{
...
public static void main(String args[]){
Person rama;
rama = new Person();
// Person rama = new Person();
rama.run();
rama.sleep();
}
}

First Program

class Hello
{
   public static void main(String args[])
   {
     System.out.println("Hello My Dear Students");
   }
}

Working with the program

• The above program is to be saved with the filename Hello.java.
• It is compiled using the command -
          javac Hello.java
• This creates a file named Hello.class (Byte Code),
• To execute the program use the command - java Hello

Next>>Data types and Operators

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