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Java this keyword


 

this keyword in Java programming language is a reference variable that refers to the current object. this in Java is  a reference to the current object, whose method is being called upon. You can use this keyword to avoid naming conflicts in the method/constructor of your object.


What are the usages of this keyword in Java Programming Language.


1) this can be used to refer the current class instance variable.


2) this can be used to invoke current class method.


3) this() method ca be used to invoke current class constructor.


4) this can be passed as an arguments in the method call.


5) this can be passed as an arguments in the constructor call.


6) this can be used to return the current instance from the method.


Lets see an example to understand how actually this keyword works..


class student

{

int roll_no;  //  variable declaration 

String Student_name; // variable declaration

float fees; //variable declaration


student(int roll_no, String Student_name, float fees) // parameterized constructor

{

roll_no = roll_no;

Student_name = Student_name;

fees = fees;

}

void display()  //java method

{

System.out.println(roll_no+" "+Student_name+" "+fees);

}

class checkingthiskeyword  // main method class

{

public static void main(String[] args)

{

student s1=new student(111,"aaa",1000f);   // creation of object 1 by alocating new memory

student s2=new student(222,"bbb",2000f);  // // creation of object 2 by alocating new memory

s1.display(); // calling method through object 1

s2.display(); // calling method through object 2

}

}

Output

0 Null 0.0

0 Null 0.0

Explanation of the program:- In the above example we have taken a class name student in which we declared instance variable in int, String, float data type then we have taken the parameterized constructor with three parameters in it, then we passed values in the parameterized constructor block then we we created a display() method in which we printed the rollno name of the student and fees then we have created the main method class in which we created object by allocating new memory the we have putted the value in the constructor itself and called the display() method through object which we have created. 

Reason why output is empty:- This is because in the parameterized constructor we have used the local variable same as of the instance variable that's why the complier throws the output as null and zero values to overcome this problem we have to use the reference of this keyword.

**)Overcoming this problem by using this keyword....

class student

{

int roll_no;  //  variable declaration 

String Student_name; // variable declaration

float fees; //variable declaration

student(int roll_no, String Student_name, float fees) // parameterized constructor

{

this.roll_no = roll_no;

this.Student_name = Student_name;

this.fees = fees;

}

void display()  //java method

{

System.out.println(roll_no+" "+Student_name+" "+fees);

}

class checkingthiskeyword  // main method class

{

public static void main(String[] args)

{

student s1=new student(111,"aaa",1000f);   // creation of object 1 by alocating new memory

student s2=new student(222,"bbb",2000f);  // // creation of object 2 by alocating new memory

s1.display(); // calling method through object 1

s2.display(); // calling method through object 2

}

}

Output

111 aaa 1000

222 bbb 2000

Explanation of the program:- In the above example we have taken a class name student in which we declared instance variable in int, String, float data type then we have taken the parameterized constructor with three parameters in it, then we passed values in the parameterized constructor block with the help of this keyword then we we created a display() method in which we printed the rollno name of the student and fees then we have created the main method class in which we created object by allocating new memory the we have putted the value in the constructor itself and called the display() method through object which we have created. 

Now we will see how this() constructor works with the help of one example.

class sample

{

sample()

{

System.out.println(" Hello welcome to java programming");

}

sample(int x)

{

this();

System.out.println(x);

}

}

class testingthis_constructor

{

public static void main(String[] args)

{

sample s1 = new sample(10);

}

}

Output

Hello welcome to java programming

10

Explanation of the program:- 

In the above example we have created a class name sample then we have taken  a default constructor in which we have printed a simple sentence as hello welcome to java programming, then we have taken a one parameterized constructor in which we have called default constructor through this() constructor then we have have the printing statement in which we will print the value that we have taken ass a parameter. Then we have taken the main method class in which we have created object by allocating the new memory and putted the value which we passed in the parameterized constructor.



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