Java this keyword - Tutorial And Example

Java this keyword

this is a reference variable in Java. It refers to the current object. It can be used inside the method or constructor of the class. this can be used to refer current object from within an instance method or constructor.

this can be used in many contexts like-

Invoke current class constructor
Invoke current class method
Return the current class object
Pass an argument in the constructor call.
Pass an argument in the method call.

Another use of this keyword is that it is not allowed two or more variable have the same name inside a class scope. Here we can see that the most common use of this keyword is to eliminate the confusion between class attributes and parameters with the same name.

Let’s have a look at below example


public class ThisExample{
  int a1;
  // Constructor with a parameter
  public ThisExample(int a1) {
    this.a1 = a1;
  }
  // Call the constructor
  public static void main(String[] args) {
    ThisExample obj = new ThisExample(5);
    System.out.println("a1 = " + obj.a1);
  }
}

public class ThisExample{

int a1;

// Constructor with a parameter

public ThisExample(int a1) {

this.a1 = a1;

}

// Call the constructor

public static void main(String[] args) {

ThisExample obj = new ThisExample(5);

System.out.println("a1 = " + obj.a1);

}

Output


a1 = 5

a1 = 5

We had used this keyword in the given example and we got output as a=5. If we omit the keyword in the given example then output will be a=0.

See the below example without this keyword.


public class  ThisExample{
  int a1;
  // Constructor with a parameter
  public ThisExample(int a1) {
    a1 = a1;
  }
  // Call the constructor
  public static void main(String[] args) {
    ThisExample obj = new ThisExample(5);
    System.out.println("a1= " + obj.a1);
  }
}

public class ThisExample{

int a1;

// Constructor with a parameter

public ThisExample(int a1) {

a1 = a1;

}

// Call the constructor

public static void main(String[] args) {

ThisExample obj = new ThisExample(5);

System.out.println("a1= " + obj.a1);

}

Output


a1 = 0

a1 = 0

‘this’ keyword in a different context

this with constructor

In the given example, ‘this’ is used to invoke the current class constructor.


class ThisExample1
{
    int a;
    int b;
  
    //Default constructor
    ThisExample1()
    {   
        this(10, 20);
        System.out.println("Inside  the  default constructor\n");
    }
    //Parameterized constructor
    ThisExample1(int a, int b)
    {
        this.a = a;
        this.b = b;
        System.out.println("Inside the parameterized constructor");
    }  
    public static void main(String[] args)
    {
        ThisExample1 obj = new ThisExample1();
    }
}

class ThisExample1

{

int a;

int b;

//Default constructor

ThisExample1()

{

this(10, 20);

System.out.println("Inside the default constructor\n");

}

//Parameterized constructor

ThisExample1(int a, int b)

{

this.a = a;

this.b = b;

System.out.println("Inside the parameterized constructor");

}

public static void main(String[] args)

{

ThisExample1 obj = new ThisExample1();

}

Output


Inside the parameterized constructor
Inside the default constructor

Inside the parameterized constructor

Inside the default constructor

Using this to return the current class instance

In this example, this keyword will use to return the current class instance.


class ThisExample2
{
    int a;
    int b;  
    //Default constructor
    ThisExample2()
    {
        a = 20;
        b = 25;
    }
    //method that return the class instance
    ThisExample2 get()
    {
        return this;
    }
    //Displaying value of variables a and b
    void display()
    {
        System.out.println("a = " + a + "  b = " + b);
    }
    public static void main(String[] args)
    {
        ThisExample2 object = new ThisExample2();
        object.get().display();
    }
}

class ThisExample2

{

int a;

int b;

//Default constructor

ThisExample2()

{

a = 20;

b = 25;

}

//method that return the class instance

ThisExample2 get()

{

return this;

}

//Displaying value of variables a and b

void display()

{

System.out.println("a = " + a + " b = " + b);

}

public static void main(String[] args)

{

ThisExample2 object = new ThisExample2();

object.get().display();

}

Output-


a = 20  b = 25

a = 20 b = 25

Using this as a method parameter-


public class ThisExample3 {
public static void main(String[] args) {
ThisExample4 obj = new ThisExample4();
obj.i = 30;
obj.method();
}
}
class ThisExample4 extends ThisExample3{
int i;
void method() {
method1(this);
}
void method1(ThisExample4 t) {
System.out.println(t.i);
}
}

public class ThisExample3 {

public static void main(String[] args) {

ThisExample4 obj = new ThisExample4();

obj.i = 30;

obj.method();

}

class ThisExample4 extends ThisExample3{

int i;

void method() {

method1(this);

}

void method1(ThisExample4 t) {

System.out.println(t.i);

}

Output-

30

Using this to invoke current class method


class ThisExample5{
     void display()
    {
        // calling fuction show()
        this.show();
        System.out.println("Inside display function");
    }
       void show() {
        System.out.println("Inside show function");
    }
       public static void main(String args[]) {
        ThisExample5 t1 = new ThisExample5();
        t1.display();
    }
}

class ThisExample5{

void display()

{

// calling fuction show()

this.show();

System.out.println("Inside display function");

}

void show() {

System.out.println("Inside show function");

}

public static void main(String args[]) {

ThisExample5 t1 = new ThisExample5();

t1.display();

}

Output-


Inside show function
Inside display function

Inside show function

Inside display function

Variable hiding in java with ‘this’ keyword

this keyword can be useful for variable hiding. In Java, we cannot take the same name for two instances; however, it is possible to take a local variable, instance variable, and mother parameter with the same name. In this process, the local variable hides the instance variable; this is called variable hiding.

Example


class ThisExample6{
int var= 10;
public static void main(String args[]) {
ThisExample6 obj = new ThisExample6();
obj.method(20);
obj.method();
}
void method(int var) {
var = 10;
System.out.println(" Val of Instance variable :" + this.var);
System.out.println("Val of Local variable :" + var);
}
void method() {
int var = 40;
System.out.println(" Val of Instance variable :" + this.var);
System.out.println("Val of Local variable :" + var);
}
}

class ThisExample6{

int var= 10;

public static void main(String args[]) {

ThisExample6 obj = new ThisExample6();

obj.method(20);

obj.method();

}

void method(int var) {

var = 10;

System.out.println(" Val of Instance variable :" + this.var);

System.out.println("Val of Local variable :" + var);

}

void method() {

int var = 40;

System.out.println(" Val of Instance variable :" + this.var);

System.out.println("Val of Local variable :" + var);

}

Output-


Val of Instance variable: 10
Val of Local variable: 10
Val of Instance variable: 10
Val of Local variable: 40

Val of Instance variable: 10

Val of Local variable: 10

Val of Instance variable: 10

Val of Local variable: 40