What is OOPS in Java?

Introduction: In this section, we will discuss the OOPs concepts along with fundamentals used to develop java applications and programs.
OOP means Object-Oriented Programming. This is a technique used to create programs around real-world entities. In OOPs programming model, programs are developed around objects and data rather than actions and logic. In OOPs, every real-life object has properties and behaviour. This feature is achieved in java through the class and object creation. They contain properties (variables of some type) and behaviour (methods). OOPs provides better flexibility and compatibility for developing large applications.

Class: A class defines the properties and behaviour (variables and methods) that is shared by all its objects. It is a blueprint for the creation of objects.

Object: Object is the basic entity of object-oriented programming language. The class itself does nothing but the real functionality is achieved through their objects. The object is an instance of the class. It takes the properties (variables) and uses the behaviour (methods) defined in the class.

Encapsulation, Inheritance, and Polymorphism are main pillars of OOPs.


These have been described below: Encapsulation: Encapsulation is the process of binding together the methods and data variables as a single entity. This keeps both the data and functionality code safe from the outside world. It hides the data within the class and makes it available only through the methods. Java provides different accessibility scopes (public, protected, private, default) to hide the data from outside.

For example, we can create a class "Check" which has a variable "amount" to store the current amount. Now to manipulate this variable we can create methods. For example, to set the value of amount create setAmount() method and to get the value of the amount to create getAmount() method. Here is the code for "Check" class:

class Check{
  private int amount=0;
  public int getAmount(){
     return amount;
  }
  public void setAmount(int amt){
     amount=amt;
  }
}

public class Mainclass{
  public static void main(String[] args){
     int amt=0;
     Check obj= new Check();
     obj.setAmount(200);
     amt=obj.getAmount();
     System.out.println("Your current amount is :"+amt);
     }
}


 

Here the data variable "amount" and methods setAmount() and getAmount() are enclosed together within a single entity called the "Check" class. These two methods are used to manipulate this variable i.e. set and get the current value of amount.

Inheritance: Inheritance allows a class (subclass) to acquire the properties and behaviour of another class (superclass). In java, a class can inherit only one class(superclass) at a time but a class can have any number of subclasses. It helps to reuse, customize and enhance the existing code. So it helps to write code accurately and reduce the development time. Java uses extends keyword to extend a class.

Example of Inheritance in Java:-

class A{
  public void fun1(int x){
     System.out.println("int in A");
  }
}
class B extends A{
  public void fun2(int x,int y){
     fun1(6);  // prints "int in A"
     System.out.println("int in B");
  }
}

public class C{
  public static void main(String[] args){
     B obj= new B();
     obj.fun2(2);
     }
}


 

In the above example, class B extends class A and so acquires properties and behaviour of class A. So we can call a method of A in class B. Polymorphism: Polymorphism allows one interface to be used for a set of actions i.e. one name may refer to different functionality. Polymorphism allows an object to accept different requests of a client (it then properly interprets the request like choosing appropriate method) and responds according to the current state of the runtime system, all without bothering the user.

There are two types of polymorphism:

Compile-time polymorphism
Runtime Polymorphism:
In compile time Polymorphism, the method to be invoked is determined at the compile time. Compile-time polymorphism is supported through the method overloading concept in java. Method overloading means having multiple methods with the same name but with the different signature (number, type, and order of parameters).

Example of polymorphism in java ?

class A{

  public void fun1(int x){
    System.out.println("int");
  }

  public void fun1(int x,int y){
    System.out.println("int and int");
  }
}

public class B{
  public static void main(String[] args)
  {
    A obj= A();
 // Here compiler decides that fun1(int) is to be called and "int" will be printed.
    obj.fun1(2);  
 // Here compiler decides that fun1(int,int) is to be called and "int and int" will be printed.
    obj.fun1(2,3);      
  }
}



 

In runtime polymorphism, the method to be invoked is determined at the run time. The example of run time polymorphism is method overriding. When a subclass contains a method with the same name and signature as in the superclass then it is called method overriding.

Example method overriding in Java?

class A{
  public void fun1(int x){
     System.out.println("int in A");
  }
  public void fun1(int x,int y){
     System.out.println("int and int");
  }
}

class C extends A{
  public void fun1(int x){
     System.out.println("int in C");
  }
}

public class D{
  public static void main(String[] args){
     A obj;
     obj= new A(); // line 1
     obj.fun1(2);  // line 2 (prints "int in A")
     obj=new C();  // line 3
     obj.fun1(2);  // line 4 (prints "int in C")
  }
}


 

In the above program, obj has been declared as A-type. Inline 1, an object of class A is assigned. Now in the next line, fun1(int) of class A will be called. Inline 3, obj has been assigned the object of class C so fun1(int) of class C will be invoked inline 4. Now we can understand that same name of the method invokes different functions, defined in different classes, according to the current type of variable "obj". This binding of method code to the method call is decided at run time.