Java Classes And Objects

In object-oriented programming technique, we design a program using objects and classes.
Object is the physical as well as logical entity whereas class is the logical entity only.

Object in Java:

Object is the real world entity that has state and behavior.
e.g. laptop, bike, dog , pen, table, car etc. It can be physical or logical.

 Car is an object. Its name is Honda City, color is Blue etc. known as its state. 

Object is an instance of a class. Class is a template or blueprint from which objects are created. So object is the instance(result) of a class.

Introduction to Java Classes:

A class is nothing but a blueprint or a template for creating different objects which defines its properties and behaviors.

A class can contain fields and methods to describe the behavior of an object.

Methods are nothing but members of a class that provide a service for an object or perform some business logic. 

Java fields and member functions names are case sensitive.

Current states of a class’s corresponding object are stored in the object’s instance variables.

Methods define the operations that can be performed in java programming.

A class has the following general syntax:

class ClassName
{
         //class body
}

Example of class

public class Rectangle
{
     int length;
     int breadth;
     int area;

     public int getArea()
     {
       area=length*breadth;
       return area;
     }
}

A class in java can contain:

data member

method

constructor

block

class and interface

How do you reference a data member/method ?

This is done by stating the name of the object reference, followed by a period (dot), followed by the name of the member inside the object.

For Example:

RectangleObject.length=20;

RectangleObject.breadth=10;

RectangleObject.getArea();

Class Variables(Static Variables)

We use class variables also know as Static variables when we want to share characteristics across all objects within a class. 

When you declare a variable as static only single copy is created among all the objects.

Hence when one object changes the value of a class variable, it affects all objects of the class. 

We can access a class variable by using the name of the class.

Static variables can be accessed even though no objects of that class exist. It is declared using static keyword.

Class Methods(Static Methods)

Class methods, similar to Class variables can be invoked without having an instance of the class.

You cannot call non-static methods from inside a static method.

Instance Variables

Each Object would have its own copy of the variable. 

When you change the value of the instance variables , it does not affects the other objects.

Top level class modifier

The modifier applicable to top level class are:

1. public

2. <default>

3. abstract

4. final

How to create Object of a class ?
By new key word we can create object of a class.New keyword occupy memory for the objects.

public class Area
{
   int length;
   int breadth;
   int area;
   public int area()
   {
      area=length*breadth;
      return area;
   }
   public static void main(String[] args)
   {
        Area a=new Area(); //object creation
        a.length=20;
        a.breadth=10;
        System.out.println("Area = " + a.area());
   }
}

OUTPUT

Area = 200