QA's approach 2 Java- This & Super Keywords

This & Super Keywords

this :

this is a reference to the current class object.this can be used with respect to variables,methods and classes.

It is a keyword.

It is a pre-defined non-static variable

It holds object reference.

It must be used inside the non-static context only.

Example 1

class Demo

{

                static

                {

                                System.out.println(this); //Compiler error.....

                }

                {

                                System.out.println(this); //No error, "this" can be used inside the non-static context....

                }

}

Question : How can we access object functionality from non-static context?

Answer : using “this”

Question : How can we access object functionality from static context?

Answer : using “user defined object reference variables”

Example 2:

class Demo

{

                Demo() //non-static context

                {

                  System.out.println(this); //it prints object reference value in Hexa string format

                }

                public static void main(String args[ ])

                {

                                new Demo( );

                }

}

Note : Generally an Object reference is +ve integer value. but that will be converted implicitly into hexa string format.

this with respect to variables:

this keyword is used to preserve the values of the instance variables.

What is an instance variable ?

A variable that is declared in the class is known as instance variable.Every object of that class will contain a copy of the variable.

Example:

class Code

{

int var=10;//instance variable

methodA()

{

----

---

}

}

Example:

import java.io.*;

class Code

{

int var=15;

methodA()

{

int var=5;

System.out.println("Local variable is"+var);

System.out.println("Instance variable is"+this.var);

}

}

class MainClass

{

public static void main(String args[])

{

Code c=new Code();

c.methodA();

}

}

Output:

Local variable is 5

Instance variable is 15

this with respect to methods:

As a general procedure,If we want to access a particular method, we create an object to that class and access the desired method. In this process 'this' is used implicitly.

Example:

import java.io.*;

class Code

{

void methodA()

{

System.out.println("Im in methodA of Code Class");

this.methodB();

}

void methodB()

{

System.out.println("Im in methodB of Code Class");

}

}

class MainClass

{

public static void main(String args[])

{

Code c=new Code();

c.methodA();



}

}

Output :

Im in methodA of Code Class

Im in methodB of Code Class

this with respect to constructors:

this keyword is used within a constructor to invoke another overloaded constructor.Usually this is placed in the first line of the calling constructor.

Example:

import java.io.*;

class Code

{

Code()

{

this(5);

System.out.println("inside the constructor without parameters");

}

Code(int a)

{

System.out.println("Inside the overloaded constructor with int value "+a);

}

public static void main(String args[])

{

Code c=new Code();//default constructor is called

}

}

Output :

Inside the overloaded constructor with int value 5

inside the constructor without parameters