this Keyword in Java

this keyword points to object's own instance. You can only use it inside the class body. Following are the usage of this keyword-

• this keyword is used to access instance variables.
• this keyword is used to call instance methods.
• this keyword is used to invoke constructor.
• You can return this keyword from the method.

Let's understand the above points with the help of examples.

To access instance variables using this keyword

If instance variables and method parameters have the same name, then this keyword helps in differentiating between instance variables and method parameters.

Problem without this keyword

public class Employee
{
    int id;
    String name;
    int salary;
    public Employee(int id, String name, int salary)
    {
	id=id;
	name=name;
	salary=salary;
    }    
    void show()
    {
    	System.out.println(id+" "+name+" "+salary);
    }
    public static void main(String args[])
    {
    	Employee emp1 = new Employee(1, "Govind", 10000);
	emp1.show();
    }
}

Output

0 null 0

As you can see method parameters and instance variables are having same name. So, this keyword is used to differentiate instance variable from method parameters.

Solution using this keyword

public class Employee
{
    int id;
    String name;
    int salary;
    public Employee(int id, String name, int salary)
    {
	this.id=id;
	this.name=name;
	this.salary=salary;
    }    
    void show()
    {
    	System.out.println(id+" "+name+" "+salary);
    }
    public static void main(String args[])
    {
    	Employee emp1 = new Employee(1, "Govind", 10000);
	emp1.show();
    }
}

Output

1 Govind 10000

To call instance method using this keyword

You can call instance method of a class with or without using this keyword.

public class X
{
	public void show()
	{
		System.out.println("Method is called using this keyword.");
	}
	public void display()
	{
		this.show();
	}
	public static void main(String args[])
	{
		X x = new X();
		x.display();
	}
}

Output

Method is called using this keyword.

To invoke constructor using this keyword

this keyword allows us calling of another constructor of the same class. If you are using this, then it must appear at the beginning of the code block of the constructor.

public class X
{
	private int a;
	public X()
	{
		this(10);
	}
	public X(int a)
	{
		this.a = a;
	}
	public void display()
	{
		System.out.println(a);
	}
	public static void main(String args[])
	{
		X x = new X();
		x.display();
	}
}

Output

10

Note- You can call any constructor from within a constructor. For example, you can call default constructor from parameterized constructor and call parameterized constructor from default constructor.

To return this keyword from the method

If you want to return this keyword from a method, then return type of the method must be a class type.

public class X
{	
	public X getX()
	{
		return this;
	}
	public void display()
	{
		System.out.println("I am a display() function.");
	}
	public static void main(String args[])
	{
		new X().getX().display();
	}
}

Output

I am a display() function.