Question

Give examples showing how "super" and "this" are useful with inheritance in Java.

**Include examples of using "super" and "this" both as constructors and as variables.**

Answer 1

Below we have implemented program to demonstrate use of "super" and "this" keywords in java and how they play a role in inheritance of classes.

Some important points about "super" and "this" :

• super is used to access methods, variables of the parent class, and this keyword is used to access methods, variables of the current class.
• super and this are first statements inside the constructor, we can use super or this as first statemtent inside constructor and not both.

Using Super in constructor:

Code:

public class Parent {

    // Parent class constructor
    public Parent() {
        System.out.println("Parent class constructor");
    }
}
class Child extends Parent{

    public Child(){

        // Calling parent class constructor
        super();
        System.out.println("From parent to child constructor");
    }

    // Main()
    public static void main(String[] args) {
        // Creating child class object
        Child c = new Child();
    }
}

Output:

Using super to access parent class variables:

Code:

public class Parent {

    // Parent class variables
    String var = "Hell0";
    int num = 25;

    // Parent class constructor
    public Parent() {
        System.out.println("Parent class constructor");
    }

}
class Child extends Parent{

    public Child(){

        // Calling parent class constructor
        super();
        System.out.println("From parent to child constructor");
    }

    public void printItems(){
        // Printing parent class variables using super
        System.out.println(super.var + " " + super.num);
    }

    // Main()
    public static void main(String[] args) {
        // Creating child class object
        Child c = new Child();
        c.printItems();
    }
}

Output:

Using this keyword to call constructors of the same class, and using this to assign values of the current class:

Code:

public class Parent {

    // Parent class variables
    private String var;
    private int num;

    // Parent class constructor
    public Parent() {

        // Using this key word to call parameterised constructor of this class
        this("Parent", 25);
        System.out.println("Parent class constructor");
    }

    public Parent(String var, int num) {

        // Using this key word to assign values to the variables of this class
        this.var = var;
        this.num = num;
    }

    public void print(){
        System.out.println(var + " " + num);
    }
}

class Child extends Parent {
    private String var;
    private int num;

    public Child() {
        // Calling Parent default constructor
        super();
    }

    public Child(String var, int num) {

        // Calling parent class constructor and passing attributes
        super(var, num);
        System.out.println("From parent to child constructor");
    }

    @Override
    public void print() {
        // Calling Parent class method using super
        super.print();
    }

    // Main()
    public static void main(String[] args) {
        // Creating child class object
        Child c = new Child("Child", 10);
        c.print();
        Child c2 = new Child();
        c2.print();
    }
}

Output:

#Please ask for any doubts. Thanks.