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Usage of this pointer

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Objectives

On completion of this period, you would be able to know, the working of :

• ‘this’ pointer• The usage of this pointer

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Recap• In the previous class, we have discussed

• Concepts of constructor

• How constructors are created ?

• Types of constructors

• Default

• Explicit

• Parameterized

• Overloaded

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• Some times a method will need to refer to the object that invoked it

• To allow this, Java defines the this keyword

• i.e. ‘this’ keyword is used to rtefer to the current object

• This can be used inside any method to refer to the current object

• i.e. this is always a reference to the object on which the method was invoked

• You can use this anywhere a reference to an object of the current class type is permitted

this Pointer

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• Broadly ‘this’ keyword can be used in the following ways

• Instance variable hiding

• To define constructors

• To pass the current object as a parameter

Usage Of this Keyword

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• It is illegal in Java to declare two local variables with the same name inside the same enclosing scopes

• You can have local variable, including formal parameters to methods, which overlap with the names of the class instance variables

• When a local variable has the same name as an instance variable, the local variable hides the instance variable

Instance Variables Hiding

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• The following example shows the formal parameter hiding the instance variables

class Point{int x,y;Point(int x, int y){

this.x = x;this.y = y;

}}

Instance Variables Hiding contd..

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• In the above example, x and y are instance variables

• In the constructor, the names of formal parameters are also x and y

• These variables hide the instance variables

• To access the instance variable use ‘this’ pointer

• In the constructor

• this.x refers to instance variable

• Simply x refers to the formal parameter

Instance Variables Hiding contd..

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• To call the constructor of the current class inside the current class ‘this’ keyword is used

• The following example shows how to do itclass Point{

int x, y;

Point(int x, int y){

this.x = x;

this.y = y;

}

Point ( ){

this(0,0);

}

}

Usage of ‘this’ In Defining Constructor

‘this’ keyword is used to call the current class’s constructor

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• In the second constructor the statement

• this(0,0);

• Calls the first constructor by passing 0, 0 as parameters

Usage of ‘this’ In Defining Constructor contd...

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• The third way to use ‘this’ keyword is to pass the current object as parameter through an instance method

• The general form of it looks like

• methodName (this);

Usage of ‘this’ In Parameter Passing contd...

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Summary

• In this class we have discussed• ‘this’ pointer• Usage of ‘this’ pointer

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1. this is a

a. Variable

b. Instance variable

c. A pointer

d. None

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Quiz

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Quiz contd...

2. this access

a. Variable

b. Instance variable

c. Constant

d. Static variable

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1. Explain the use of this pointer with an example program

2. How we can hide the instance variables using this pointer?

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Frequently Asked Questions

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1. Write a class that implements this pointer

2. Create a Student class that calls its constructor using ‘this’ keyword

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Assignment