In Java, this keyword is used to refer to the current object inside a method or a constructor. For example,
class Main {
int instVar;
Main(int instVar){
this.instVar = instVar;
System.out.println("this reference = " + this);
}
public static void main(String[] args) {
Main obj = new Main(8);
System.out.println("object reference = " + obj);
}
}
Output:
this reference = Main@23fc625e object reference = Main@23fc625e
In the above example, we created an object named obj of the class Main. We then print the reference to the object obj and this keyword of the class.
Here, we can see that the reference of both obj and this is the same. It means this is nothing but the reference to the current object.
Use of this Keyword
There are various situations where this keyword is commonly used.
1. Using this for Ambiguity Variable Names
In Java, it is not allowed to declare two or more variables having the same name inside a scope (class scope or method scope). However, instance variables and parameters may have the same name. For example,
class MyClass {
// instance variable
int age;
// parameter
MyClass(int age){
age = age;
}
}
In the above program, the instance variable and the parameter have the same name: age. Here, the Java compiler is confused due to name ambiguity.
In such a situation, we use this keyword. For example,
First, let's see an example without using this keyword:
class Main {
int age;
Main(int age){
age = age;
}
public static void main(String[] args) {
Main obj = new Main(8);
System.out.println("obj.age = " + obj.age);
}
}
Output:
obj.age = 0
In the above example, we have passed 8 as a value to the constructor. However, we are getting 0 as an output. This is because the Java compiler gets confused because of the ambiguity in names between instance the variable and the parameter.
Now, let's rewrite the above code using this keyword.
class Main {
int age;
Main(int age){
this.age = age;
}
public static void main(String[] args) {
Main obj = new Main(8);
System.out.println("obj.age = " + obj.age);
}
}
Output:
obj.age = 8
Now, we are getting the expected output. It is because when the constructor is called, this inside the constructor is replaced by the object obj that has called the constructor. Hence the age variable is assigned value 8.
Also, if the name of the parameter and instance variable is different, the compiler automatically appends this keyword. For example, the code:
class Main {
int age;
Main(int i) {
age = i;
}
}
is equivalent to:
class Main {
int age;
Main(int i) {
this.age = i;
}
}
2. this with Getters and Setters
Another common use of this keyword is in setters and getters methods of a class. For example:
class Main {
String name;
// setter method
void setName( String name ) {
this.name = name;
}
// getter method
String getName(){
return this.name;
}
public static void main( String[] args ) {
Main obj = new Main();
// calling the setter and the getter method
obj.setName("Toshiba");
System.out.println("obj.name: "+obj.getName());
}
}
Output:
obj.name: Toshiba
Here, we have used this keyword:
- to assign value inside the setter method
- to access value inside the getter method
3. Using this in Constructor Overloading
While working with constructor overloading, we might have to invoke one constructor from another constructor. In such a case, we cannot call the constructor explicitly. Instead, we have to use this keyword.
Here, we use a different form of this keyword. That is, this(). Let's take an example,
class Complex {
private int a, b;
// constructor with 2 parameters
private Complex( int i, int j ){
this.a = i;
this.b = j;
}
// constructor with single parameter
private Complex(int i){
// invokes the constructor with 2 parameters
this(i, i);
}
// constructor with no parameter
private Complex(){
// invokes the constructor with single parameter
this(0);
}
@Override
public String toString(){
return this.a + " + " + this.b + "i";
}
public static void main( String[] args ) {
// creating object of Complex class
// calls the constructor with 2 parameters
Complex c1 = new Complex(2, 3);
// calls the constructor with a single parameter
Complex c2 = new Complex(3);
// calls the constructor with no parameters
Complex c3 = new Complex();
// print objects
System.out.println(c1);
System.out.println(c2);
System.out.println(c3);
}
}
Output:
2 + 3i 3 + 3i 0 + 0i
In the above example, we have used this keyword,
- to call the constructor
Complex(int i, int j)from the constructorComplex(int i) - to call the constructor
Complex(int i)from the constructorComplex()
Notice the line,
System.out.println(c1);
Here, when we print the object c1, the object is converted into a string. In this process, the toString() is called. Since we override the toString() method inside our class, we get the output according to that method.
One of the huge advantages of this() is to reduce the amount of duplicate code. However, we should be always careful while using this().
This is because calling constructor from another constructor adds overhead and it is a slow process. Another huge advantage of using this() is to reduce the amount of duplicate code.
Note: Invoking one constructor from another constructor is called explicit constructor invocation.
Passing this as an Argument
We can use this keyword to pass the current object as an argument to a method. For example,
class ThisExample {
// declare variables
int x;
int y;
ThisExample(int x, int y) {
// assign values of variables inside constructor
this.x = x;
this.y = y;
// value of x and y before calling add()
System.out.println("Before passing this to addTwo() method:");
System.out.println("x = " + this.x + ", y = " + this.y);
// call the add() method passing this as argument
add(this);
// value of x and y after calling add()
System.out.println("After passing this to addTwo() method:");
System.out.println("x = " + this.x + ", y = " + this.y);
}
void add(ThisExample o){
o.x += 2;
o.y += 2;
}
}
class Main {
public static void main( String[] args ) {
ThisExample obj = new ThisExample(1, -2);
}
}
Output:
Before passing this to addTwo() method: x = 1, y = -2 After passing this to addTwo() method: x = 3, y = 0
In the above example, inside the constructor ThisExample(), notice the line,
add(this);
Here, we are calling the add() method by passing this as an argument. Since this keyword contains the reference to the object obj of the class, we can change the value of x and y inside the add() method.