this keyword in java

→ this keyword is used to refer to the current object inside a method or a constructor etc.

Usage of this keyword.

Instance variable.
Method
Constructor.
Passed as an argument in the method call.
Passed as argument in the constructor call.
Return the current class instance from the method.

1) this keyword used in Instance variable

→ this keyword can be used to refer current class instance variable.

Example 1 : by this keyword

 class Student{  
int rollno;  
String name;  
float fee;  
Student(int rollno,String name,float fee){  
this.rollno=rollno;  
this.name=name;  
this.fee=fee;  
}  
void display(){System.out.println(rollno+" "+name+" "+fee);}  
}  
class TestS{
public static void main(String args[]){  
Student s1=new Student(1,"Janvi",5000f);  
Student s2=new Student(112,"Smith",6000f);  
s1.display();  
s2.display();  
}}  

Output

Example 2 : problem without this keyword

 class Student{  
int rollno;  
String name;  
float fee;  
Student(int rollno,String name,float fee){  
rollno=rollno;  
name=name;  
fee=fee;  
}  
void display(){System.out.println(rollno+" "+name+" "+fee);}  
}  
class TestS{
public static void main(String args[]){  
Student s1=new Student(1,"Janvi",5000f);  
Student s2=new Student(112,"Smith",6000f);  
s1.display();  
s2.display();  
}}  
 

Output

2) this keyword used in Method

→ this keyword can be used to invoke the method of the current class.If you don't use the this keyword, compiler automatically adds this keyword while invoking the method.

Example :

 class A{  
void m(){System.out.println("hello m");}  
void n(){  
System.out.println("hello n");  
//m();//same as this.m()  
this.m();  
}  
}  
class TestExample{  
public static void main(String args[]){  
A a=new A();  
a.n();  
}} 

Output

3) this keyword used in Constructor

→ this() constructor call can be used to invoke the current class constructor. It is used to reuse the constructor. In other words, it is used for constructor chaining.

Example :

 class Student{
   int id;
   String name;
   String course;
   Student() {
   System.out.println("Default Constructor");
  }
  Student(int id,String name,String course){
  this();
  this.id=id;
  this.name=name;
  this.course= course;
  System.out.println("Print Our Program");
  }
 void show(){
  System.out.println(id+ " "+name+ " "+course);
 }
  public static void main (String arg[]){
   Student s= new Student(01,"Pradeep","MCA");
   s.show();
}
 }

Output

4) Pass as an argument in the method

→ this keyword can also be passed as an argument in the method. It is mainly used in the event handling.

Example :

 class ArgumentPass{  
          void m(ArgumentPass obj){  
        System.out.println("method is print");  
    }  
      void p(){  
         m(this);  
      }  
      public static void main(String args[]){  
    ArgumentPass s1 = new ArgumentPass();  
  s1.p();  
  }  
}  

Output

Pass as an argument in the method Example in java

5) Passed as argument in the constructor call.

We pass the this keyword in the constructor also. It is useful if we have to use one object in multiple classes.

Example :

  class B{  
  A4 obj;  
  B(A4 obj){  
    this.obj=obj;  
  }  
  void display(){  
    System.out.println(obj.data);//using data member of A4 class  
  }  
}  
  
class A4{  
  int data=10;  
  A4(){  
   B b=new B(this);  
   b.display();  
  }  
  public static void main(String args[]){  
   A4 a=new A4();  
  }  
}  

Output

6) Return the current class instance from the method..

We can return this keyword as an statement from the method.

Syntax :

  return_type method_name(){  
return this;  
}  
  

Example :

    class A{  
   A getA(){  
  return this;  
}  
void msg(){System.out.println("Hello java");}  
}  
class Test{  
public static void main(String args[]){  
new A().getA().msg();  
}  
}  

Output

Return the current class instance from the method in java

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OOPs Concepts

this keyword in java

Usage of this keyword.

1) this keyword used in Instance variable

2) this keyword used in Method

3) this keyword used in Constructor

4) Pass as an argument in the method

5) Passed as argument in the constructor call.

6) Return the current class instance from the method..