this keyword in Java

There can be a lot of usage of this keyword. In java, this is a reference variable that refers to the current object. Within an instance method or a constructor, this is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this.
Usage of this keyword
Here is given the 6 usage of this keyword.
  1. this keyword can be used to refer current class instance variable.
  2. this() can be used to invoke current class constructor.
  3. this keyword can be used to invoke current class method (implicitly)
  4. this can be passed as an argument in the method call.
  5. this can be passed as argument in the constructor call.
  6. this keyword can also be used to return the current class instance.
Using this with a Field

The most common reason for using the this keyword is because a field is shadowed by a method or constructor parameter.

For example, the Point class was written like this
public class Point {
    public int x ;
    public int y ;
        
    //constructor
    public Point(int x, int y) {
        x = x;
        y = y;
    }
   void display(){
      System.out.println(x+" :: "+y);
   }
   public static void main(String args[]){
    Point s1 = new Point (111,222);
    Point s2 = new Point (333,444);
    s1.display();
    s2.display();
    }
}

output:
0 :: 0
0 :: 0

In the above example, parameter (formal arguments) and instance variables are same that is why we are using this keyword to distinguish between local variable and instance variable.

but it could have been written like this:
public class Point {
    public int x = 0;
    public int y = 0;
        
    //constructor
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
   void display(){
      System.out.println(x+" :: "+y);
   }
   public static void main(String args[]){
    Point s1 = new Point (111,222);
    Point s2 = new Point (333,444);
    s1.display();
    s2.display();
    }
}

output:
111 :: 222
333 :: 444


Using this with a Constructor

From within a constructor, you can also use the this keyword to call another constructor in the same class. Doing so is called an explicit constructor invocation. Here's another Rectangle class, with a different implementation from the one in the Objects section.
public class Rectangle {
    private int x, y;
    private int width, height;
        
    public Rectangle() {
        this(0, 0, 0, 0);
    }
    public Rectangle(int width, int height) {
        this(0, 0, width, height);
    }
    public Rectangle(int x, int y, int width, int height) {
        this.x = x;
        this.y = y;
        this.width = width;
        this.height = height;
    }
    ...
}
This class contains a set of constructors. Each constructor initializes some or all of the rectangle's member variables. The constructors provide a default value for any member variable whose initial value is not provided by an argument. For example, the no-argument constructor calls the four-argument constructor with four 0 values and the two-argument constructor calls the four-argument constructor with two 0 values. As before, the compiler determines which constructor to call, based on the number and the type of arguments.

If present, the invocation of another constructor must be the first line in the constructor.

Where to use this() constructor call?
The this() constructor call should be used to reuse the constructor in the constructor. It maintains the chain between the constructors i.e. it is used for constructor chaining. Let's see the example given below that displays the actual use of this keyword.
class Employee{
    int id;
    String name;
    String city;
    
    Employee(int id,String name){
    this.id = id;
    this.name = name;
    }
    Employee(int id,String name,String city){
    this(id,name);//now no need to initialize id and name
    this.city=city;
    }
    void display(){
     System.out.println(id+" "+name+" "+city);
    }
    
    public static void main(String args[]){
    Employee e1 = new Employee(222,"Dinesh");
    Employee e2 = new Employee(555,"Anamika","Noida");
    e1.display();
    e2.display();
   }
}

Output:
222 Dinesh null
555 Anamika Noida

Note: Call to this() must be the first statement.
class Employee{
    int id;
    String name;
    Employee(){
     System.out.println("default constructor is invoked");
    }
    
    Employee(int id,String name){
    id = id;
    name = name;
    this ();//must be the first statement
    }
    void display(){
     System.out.println(id+" "+name);
   }
    
    public static void main(String args[]){
    Employee e1 = new Employee(222,"Dinesh");
    Employee e2 = new Employee(555,"Anamika");
    e1.display();
    e2.display();
   }
}
Output:
Compile Time Error



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