What is Object And Class ?

Object And Class in  JAVA

In this page, we will learn about Java objects and classes. 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.
An entity that has state and behavior is known as an object e.g. chair, bike, pen, table, car
It can be physical or logical (tengible and intengible).The example of integible object is banking system.

An object has three characteristics:

state: represents data (value) of an object.
behavior: represents the behavior (functionality) of an object such as deposit, withdraw etc.
identity: Object identity is typically implemented via a unique ID. The value of the ID is not visible to the external user.But,it is used internally by the JVM to identify each object uniquely.

For Example: Car is an object. Its name is Safari, color is black etc. known as its state. It is used to drive, so driving is its behavior.

Class in JAVA.

A class is a group of objects that has common properties. It is a template or blueprint from which objects are created.

A class in java can contain:

1.data member
2.method
3.constructor
4.block
5.class and interface


Syntax of class:

class <class_name>{
data member;
method;
}

Simple Example of Object and Class:

In this example, we have created a Student class that have two data members id and name.We are creating the object of the Student class by new keyword and printing  the objects value.

class Student{
int id; //data member (also instance variable)
String name; //data member(also instance variable)

public static void main(String []args){
Student s1=new Student();  //creating an object of Student
System.out.println(s1.id+" "+s1.name);
  }
}

Output: 0 null

What is Method in Java?

In java, a method is like function i.e. used to expose behaviour of an object.

Example of Method:

class Student
{ String name="vishwa";
void show()  //here show() is method name
{
System.out.println(name);
 }
public static void main(String []args)
{
Student s1=new Student();
s1.show();
  }
}

Output: vishwa

Advantage of Method:

>Code Reusability
>Code Optimization

What is new keyword in Java?

The new is a keyword that is used to allocate memory at runtime.

Example of Object and class that maintains the records of  students using method:

In this example, we are creating the two objects of Student  class and initializing the value to these objects by invoking the insert method on it. Here, we are displaying the
state (data) of the objects by invoking the display method.

class Student{
int rollno;
String name;
void insert(int r, String n){ //method
rollno=r;
name=n;
}
void display() { System.out.println(rollno+" "+name);} //method

public static void main(String []args){
Student s1=new Student();
Student s2=new Student();
s1.insert(123,"vishwa");
s2.insert(456,"dolly");
s1.display();
s2.display();
    }
}

Output:123 vishwa
            456 dolly