Core Java

this keyword in Java

What is this

this is a keyword in Java. It can be used inside the method or constructor of a class. It(this) works as a reference to the current object, whose method or constructor is being invoked. This keyword can refer to any member of the current object from within an instance method or a constructor.

this keyword with a field(Instance Variable)

this keyword can be handy in the handling of Variable Hiding. We can not create two instances/local variables with the same name. However, it is legal to create one instance variable & one local variable or Method parameter with the same name. In this scenario, the local variable will hide the instance variable; this is called Variable Hiding.

Example of Variable Hiding

class JBT {

	int variable = 5;

	public static void main(String args[]) {
		JBT obj = new JBT();

		obj.method(20);
		obj.method();
	}

	void method(int variable) {
		variable = 10;
		System.out.println("Value of variable :" + variable);
	}

	void method() {
		int variable = 40;
		System.out.println("Value of variable :" + variable);
	}
}

The output of the above program

Value of variable :10
Value of variable :40

As you can see in the example above, the instance variable is hiding, and the value of the local variable (or Method Parameter) is displayed, not the instance variable. To solve this problem, use this keyword to point to the instance variable instead of the local variable.

Example of this keyword in Java for Variable Hiding

class JBT {

	int variable = 5;

	public static void main(String args[]) {
		JBT obj = new JBT();

		obj.method(20);
		obj.method();
	}

	void method(int variable) {
		variable = 10;
		System.out.println("Value of Instance variable :" + this.variable);
		System.out.println("Value of Local variable :" + variable);
	}

	void method() {
		int variable = 40;
		System.out.println("Value of Instance variable :" + this.variable);
		System.out.println("Value of Local variable :" + variable);
	}
}

The output of the above program

Value of Instance variable :5
Value of Local variable :10
Value of Instance variable :5
Value of Local variable :40

this Keyword with Constructor

“this” keyword can be used inside the constructor to call another overloaded constructor in the same class. It is called the Explicit Constructor Invocation. This occurs if a Class has two overloaded constructors, one without argument and another with the argument. Then this” keyword can call the constructor with an argument from the constructor without argument. This is required as the constructor cannot be called explicitly.

Example of this with Constructor

class JBT {

	JBT() {
		this("JBT");
		System.out.println("Inside Constructor without parameter");
	}

	JBT(String str) {
		System.out
				.println("Inside Constructor with String parameter as " + str);
	}

	public static void main(String[] args) {
		JBT obj = new JBT();
	}
}

The output of the above program

Inside Constructor with String parameter as JBT
Inside Constructor without parameter

As you can see, “this” can invoke an overloaded constructor in the same class.

Note*:

  • this keyword can only be the first statement in Constructor.
  • A constructor can have either this or super keyword but not both.

this Keyword with Method

this keyword can also be used inside Methods to call another Method from the same Class.

Example of this keyword with Method

class JBT {

	public static void main(String[] args) {
		JBT obj = new JBT();
		obj.methodTwo();
	}
	void methodOne(){
		System.out.println("Inside Method ONE");
		}

	void methodTwo(){
		System.out.println("Inside Method TWO");
		this.methodOne();// same as calling methodOne()
	}
}

The output of the above program

Inside Method TWO
Inside Method ONE

Example of this keyword as a Method parameter

public class JBTThisAsParameter {

	public static void main(String[] args) {
		JBT1 obj = new JBT1();
		obj.i = 10;
		obj.method();
	}

}

class JBT1 extends JBTThisAsParameter {
	int i;

	void method() {
		method1(this);
	}

	void method1(JBT1 t) {
		System.out.println(t.i);
	}
}

If you understood this keyword correctly, the next step should be to understand the Static Keyword in Java from Java Tutorial.

References
1- Official Document

Author

70 thoughts

  1. Great and complete explanation of the word “this”. Something basic that many faculty professors wont tell their students…

  2. It is very important concept in java .This keyword is a reference variable that refer the current object in java. This keyword can be used for call current class constructor.

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