Keyword : this

“this” is a reference to the current object. i.e, by using “this” in any instance method or constructor, you can point out current object properties, behaviors and you can call current object constructors.

Example

class Person{
 String firstName, lastName;

 Person(String firstName, String lastName){
  this.firstName = firstName;
  this.lastName = lastName;
 }

 public static void main(String args[]){
  Person p1 = new Person("Gopi", "Battu");
  System.out.println(p1.firstName +"\t"+p1.lastName);
 }
}

Output    

Gopi Battu

As you seen in the above example, constructor Person has parameter names as firstName, lastName which are same like instance properties of an object. So to differentiate parameters with instance properties we used the keyword “this”.

this can be used to call constructor

    Example

class Person{
 String firstName, lastName;

 Person(){
  this("noName", "noName");  // calls the constructor
 }

 Person(String firstName, String lastName){
  this.firstName = firstName;
  this.lastName = lastName;
 }

 public static void main(String args[]){
  Person p1 = new Person();
  System.out.println(p1.firstName +"\t"+p1.lastName);
 }
}

Output

 noName noName

        

As you see in the above program, this is used to call the constructor.

Some Points to Remember

1. call to “this” must be first statement in constructor

     Example

class Person{
 String firstName, lastName;

 Person(){
  firstName = "HI";
  this("noName", "noName");
 }

 Person(String firstName, String lastName){
  this.firstName = firstName;
  this.lastName = lastName;
 }

 public static void main(String args[]){
  Person p1 = new Person();
  System.out.println(p1.firstName +"\t"+p1.lastName);
 }
}

When you try to compile the above program, compiler throws below error

    

Person.java:7: error: call to this must be first statement in constructor
this("noName", "noName");
 ^
1 error

    

2. Calling this from any methods other than constructor causes compiler error         

class Person{
 Person(){

 }

 void setName(){
  this();
 }
}

         

When you try to compile the above program, compiler throws below error

Person.java:7: error: call to this must be first statement in constructor
     this();
     ^
     1 error

              

        

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