super and this key words in Java

Hi All,

Welcome to Java-recent.

In this post we will discuss about usage of “super” and “this” key words .

First we will discuss about super keyword

super:-

• Is a key word in Java primarily used to invoke immediate super /parent class methods
• used to invoke immediate super/parent class constructor
• used to invoke immediate super/parent class instance variable

We will use same old classes Animal.java , Lion.java

First lets have a look at the code

Animal.java

public class Animal {

private String name="java-recent";

//Parent class parameterized constructor

public Animal(String animal)

{

System.out.println(animal);

}

//Display method

public void display()

{

System.out.println("in animal-a parent class");

}

}

Lion.java

public class Lion extends Animal{

public Lion()

{

super("From lion");

System.out.println("In child class Lion constructor");

}

public void display()

{

System.out.println("i am an Elephant-a child class");

//calling Animal class display method

super.display();

//Invoking super class instance report

System.out.println(super.name);

}

}

Tester.java

public class Tester

{

public static void main(String[] args)

{

//creating Lion object

Lion lion=new Lion();

lion.display();

}

}

output

From Lion

In child class Lion constructor

i am an Elephant-a child class

in animal-a parent class

java-recent

Example :-

1. Invoking of parent class super method using super.display() in Lion.java;
   output for it is in animal-a parent class
2. Invoking super class constructor
   super(“From lion”); in Lion constructor
   output for this is “From Lion” statement
3. Invoking super class instance variable
   super.name is used to access super class instance variable
   output for this is “java-recent”

Note:-

• super statement in child class constructor should be the first statement
• “ If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the super class does not have a no-argument constructor, you will get a compile-time error. Object does have such a constructor, so if Object is the only superclass, there is no problem. “ Taken form http://docs.oracle.com/javase/tutorial/java/IandI/super.html

Now we will discuss about this key word

“this”

• this key word is used to refer members of same class.
• this is reference to current object

We will see the importance of this key word in the following examples.

public class ThisKey

{

private int height;

private int width;

public ThisKey(int height,int width )

{

height=height;

width=width;

}

//Getters and setters

public int getHeight() {

return height;

}

public void setHeight(int height) {

this.height = height;

}

public int getWidth() {

return width;

}

public void setWidth(int width) {

this.width = width;

}

public static void main(String[] args)

{

ThisKey thisKey=new ThisKey(10,20);

System.out.println("height is "+thisKey.getHeight());

System.out.println("width is "+thisKey.getWidth());

}

}

Output:-

What would be the output values for height and width 10,20 ?

No the output will be zero and zero. How ?

In the parametrized constructor we are using height=height and width=width, according to variable scope local variables will have more importance,so the assignment refers to local values,instance values are not updated.

height is 0

width is 0

Now if we want to assign values for instance variable we need to use this key word.

Just change the code of constructor as below

public ThisKey(int height,int width )

{

this.height=height;

this.width=width;

}

Output:- As we has used this key word,the value will be assigned to instance variables

height is 10

width is 20

Similarly we can use this() for calling self constructors/methods

Examples:-

public ThisKey(int height,int width )

{ //calling self constructor

this();

this.height=height;

this.width=width;

}

//default constructor

public ThisKey()

{

}

         Happy Learning

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