Objects and Classes in Java Language

Java classes and objects will be covered on this page.

We create programs utilizing objects and classes while using object-oriented programming techniques.

In Java, an object is both a physical and a logical thing, whereas a class is a logical entity only.

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Objects

An object in Java is an instance of a class, which is a blueprint for creating objects. The building blocks of object-oriented programming (OOP) are objects, which stand in for actual people, things, ideas, or data structures.

Characteristics of an object:

There are three characteristics of an object as follows

• State :

  Objects store data or attributes that define the current state.

• Behavior :

  Objects can perform actions or exhibit behavior through methods. Methods define what an object can do and how it responds to certain actions or requests. such as deposits, withdrawals, etc.

• Identity :

  Each object has a unique identity, which is typically represented by its memory address or reference. This identity distinguishes one object from another.

Note:- An instance of a class is an object. An object can be produced from a class, which functions as a template or blueprint. Consequently, an object is a class instance.

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Classes

A class is like a blueprint or template for creating objects. It defines the structure and behavior that objects of that class will have. Think of it as a recipe for making cookies, the class is the recipe, and the cookies you bake from it are the objects.

Java class can contain :

• Fields

• Methods

• Blocks

• Constructors

• Inner Classes

• Access modifiers

• Static members, etc.

Syntax to declare a class :

class <class_name>
{  
    variables;  
    method;  
}

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3 Ways to Initialize an object

There are three ways to initialize objects in Java

• By reference variable

• By method

• By constructor

1. Through reference Example:

Initializing means storing data in an object. So let's see how we can apply the first method, through an example

Filename:- Test.java

class Student{  
 int id;  
 String name;  
}  
class Test{  
 public static void main(String[] args){  
  Student obj = new Student();  
  obj.id=101;  
  obj.name="Kartik";  
  System.out.println(obj.id+" "+obj.name);//printing members with a white space  
 }  
}

Output :

101 Kartik

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1. Through Method Example :

In this example, we are creating the two objects of the Student class and initializing the value to these objects by invoking the insert method.

Filename: Test.java

class Student{  
 int rollno;  
 String name;  
 void insert(int r, String n){  
  rollno=r;  
  name=n;  
 }  
 void display(){System.out.println(rollno+" "+name);}  
}  
class Test{  
 public static void main(String args[]){  
  Student obj_1=new Student();  
  Student obj_2=new Student();  
  obj_1.insert(101,"Kartik Sharma");  
  obj_2.insert(102,"Aman Bisht");  
  obj_1.displal();  
  obj_2.display();  
 }  
}

Output :

101 Kartik Sharma
102 Aman Bisht

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1. Through Constructor Example :

we can pass value with the help of constructor and we use parameterized constructor.

Filename: Test.java

class Student{
int id;
String name;
// Constructor
Student(int i, String n){
id = i;
name = n;
} 
void display(){
System.out.println(id+" "+name);
}
}

class Test{
public static void main(String[] args){
 Student obj = new Student(101,"Kartik"); //this is a parameterized constructor 
obj.display(); // Calling display method
}
}

Output :

101 Kartik