This operator in java with example

this keyword in Java

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 in java 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.

• 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

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Java Static Keyword

70 thoughts on “this keyword in Java”

class car
int speed;
car()
speed=70;
>
protected void finalize()
System .out.println(“destroy object of car”);
>
>
class bike
int speed;
bike()
speed=70;
>
protected void finalize()
System.out.println(“destroy object of bike”)
>
>
class Garbage_Collection
public static void main(String []args)
car c=new car();
bike b=new bike();
b=null;
System.gc();
System.out.println(“speed of car: “+c.speed);
>
>

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()
>
> when I removed ‘this’ from the line this.methodOne();// same as calling methodOne() I get the same output. Is this the standard behaviour ?

Yes, that is standard behavior. this is just used to point to the current object and in this case, it is the current object’s method. So it will get called.

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Using the this Keyword

Within an instance method or a constructor, this is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this .

Using this with a Field

The most common reason for using the this keyword is because a field is shadowed by a method or constructor parameter.

For example, the Point class was written like this

but it could have been written like this:

Each argument to the constructor shadows one of the object’s fields — inside the constructor x is a local copy of the constructor’s first argument. To refer to the Point field x , the constructor must use this.x .

Using this with a Constructor

From within a constructor, you can also use the this keyword to call another constructor in the same class. Doing so is called an explicit constructor invocation. Here’s another Rectangle class, with a different implementation from the one in the Objects section.

public class Rectangle < private int x, y; private int width, height; public Rectangle() < this(0, 0, 1, 1); > public Rectangle(int width, int height) < this(0, 0, width, height); > public Rectangle(int x, int y, int width, int height) < this.x = x; this.y = y; this.width = width; this.height = height; >. >

This class contains a set of constructors. Each constructor initializes some or all of the rectangle’s member variables. The constructors provide a default value for any member variable whose initial value is not provided by an argument. For example, the no-argument constructor creates a 1×1 Rectangle at coordinates 0,0. The two-argument constructor calls the four-argument constructor, passing in the width and height but always using the 0,0 coordinates. As before, the compiler determines which constructor to call, based on the number and the type of arguments.

If present, the invocation of another constructor must be the first line in the constructor.

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this Keyword in Java: What is & How to use with Example

this keyword in Java is a reference variable that refers to the current object of a method or a constructor. The main purpose of using this keyword in Java is to remove the confusion between class attributes and parameters that have same names.

Following are various uses of ‘this’ keyword in Java:

• It can be used to refer instance variable of current class
• It can be used to invoke or initiate current class constructor
• It can be passed as an argument in the method call
• It can be passed as argument in the constructor call
• It can be used to return the current class instance

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Understand ‘this’ keyword with an example.

1. Class: class Account
2. Instance Variable: a and b
3. Method Set data: To set the value for a and b.
4. Method Show data: To display the values for a and b.
5. Main method: where we create an object for Account class and call methods set data and show data.

Let’s compile and run the code

Our expected output for A and B should be initialized to the values 2 and 3 respectively.

But the value is 0, Why? Let investigate.

In the method Set data, the arguments are declared as a and b, while the instance variables are also named as a and b.

During execution, the compiler is confused. Whether “a” on the left side of the assigned operator is the instance variable or the local variable. Hence, it does not set the value of ‘a’ when the method set data is called.

The solution is the “this” keyword

Append both ‘a’ and ‘b’ with the Java this keyword followed by a dot (.) operator.

During code execution when an object calls the method ‘setdata’. The keyword ‘this’ is replaced by the object handler “obj.” (See the image below).

So now the compiler knows,

• The ‘a’ on the left-hand side is an Instance variable.
• Whereas the ‘a’ on right-hand side is a local variable

The variables are initialized correctly, and the expected output is shown.

Suppose you are smart enough to choose different names for your instance variable and methods arguments.

But this time around, you create two objects of the class, each calling the set data method.

How the compiler will determine whether it is supposed to work on instance variable of object 1 or object 2.

Well, the compiler implicitly appends the instance variable with “this” keyword (image below).

Such that when object 1 is calling the set data method, an instance variable is appended by its reference variable.

While object 2 is calling the set data method, an instance variable of object 2 is modified.

This process is taken care by the compiler itself. You don’t have to append ‘this’ keyword explicitly unless there is an exceptional situation as in our example.

Example: To learn the use “this” keyword

Step 1) Copy the following code into a notepad.

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this keyword in java with example

this keyword in java is used to refer to current object or instance of class. It can be used in the constructor to call any other overloaded constructor but this keyword should be the first statement in the constructor.

This keyword can be used for instance variables

this keyword can be used to refer to the instance variable of class.

When you run above program, you will get below output:

As you can see we have used this keyword to set values for instance variable in the constructor.

This keyword can be used to call the overloaded constructor

If you want to call overloaded constructor of same class, you can use this keyword to do that.
For example:

When you run above program, you will get below output:

Calling No arg constructor
Calling Parameterized constructor
Employee’s name : John
Employee’s age : 20

Please note that this keyword used for calling another constructor should be first statement in that constructor.

this keyword can be used return object of the class

that’s all about this keyword in java.

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