Java this Keyword

Java `this` Keyword

The this keyword in Java is a reference variable that refers to the current object. It is mainly used to differentiate instance variables from local variables and to call constructors or methods of the current class.

1. Uses of `this` Keyword

Referring to Current Object
Accessing Instance Variables (when shadowed by local variables)
Calling Another Constructor (constructor chaining)
Passing Current Object as a Parameter
Calling Current Class Methods

2. Example 1: Accessing Instance Variables

class Person {
String name;

void setName(String name) {
this.name = name; // 'this.name' refers to instance variable
}

void display() {
System.out.println("Name: " + name);
}
}

public class Main {
public static void main(String[] args) {
Person p = new Person();
p.setName("Alice");
p.display();
}
}

class Person {

String name;

void setName(String name) {

this.name = name; // 'this.name' refers to instance variable

}

void display() {

System.out.println("Name: " + name);

}

public class Main {

public static void main(String[] args) {

Person p = new Person();

p.setName("Alice");

p.display();

}

📌 Output:

Without this, name = name; would refer to the local parameter, not the instance variable.

3. Example 2: Calling Another Constructor (Constructor Chaining)

class Car {
String brand;
int year;

Car() {
this("Unknown", 0); // call parameterized constructor
}

Car(String brand, int year) {
this.brand = brand;
this.year = year;
}

void display() {
System.out.println("Brand: " + brand + ", Year: " + year);
}
}

public class Main {
public static void main(String[] args) {
Car car1 = new Car();
Car car2 = new Car("Toyota", 2022);

car1.display();
car2.display();
}
}

class Car {

String brand;

int year;

Car() {

this("Unknown", 0); // call parameterized constructor

}

Car(String brand, int year) {

this.brand = brand;

this.year = year;

}

void display() {

System.out.println("Brand: " + brand + ", Year: " + year);

}

public class Main {

public static void main(String[] args) {

Car car1 = new Car();

Car car2 = new Car("Toyota", 2022);

car1.display();

car2.display();

}

📌 Output:

4. Example 3: Passing Current Object as Parameter

class Person {
String name;

Person(String name) {
this.name = name;
}

void display() {
System.out.println("Name: " + name);
}

void greet(Person p) {
System.out.println("Hello, " + p.name);
}

void greetCurrent() {
greet(this); // pass current object
}
}

public class Main {
public static void main(String[] args) {
Person p = new Person("Alice");
p.greetCurrent();
}
}

class Person {

String name;

Person(String name) {

this.name = name;

}

void display() {

System.out.println("Name: " + name);

}

void greet(Person p) {

System.out.println("Hello, " + p.name);

}

void greetCurrent() {

greet(this); // pass current object

}

public class Main {

public static void main(String[] args) {

Person p = new Person("Alice");

p.greetCurrent();

}

📌 Output:

5. Example 4: Calling Current Class Method

class Calculator {
int add(int a, int b) {
return a + b;
}

void displaySum(int a, int b) {
System.out.println("Sum = " + this.add(a, b)); // call method of current object
}
}

public class Main {
public static void main(String[] args) {
Calculator calc = new Calculator();
calc.displaySum(5, 10);
}
}

class Calculator {

int add(int a, int b) {

return a + b;

}

void displaySum(int a, int b) {

System.out.println("Sum = " + this.add(a, b)); // call method of current object

}

public class Main {

public static void main(String[] args) {

Calculator calc = new Calculator();

calc.displaySum(5, 10);

}

📌 Output:

6. Key Points About `this`

Usage	Purpose
`this.variable`	Refers to current object’s instance variable
`this()`	Calls another constructor in the same class
`this.method()`	Calls another method of the current object
`this` as parameter	Pass current object to another method or constructor

Helps resolve ambiguity between local and instance variables.
Can be used for constructor chaining but must be the first statement in a constructor.

Java this Keyword