Using super keyword

You can use super in a subclass to refer to its immediate superclass. super has two general forms.

The first calls the superclass' constructor.
The second is used to access a member of the superclass.

Using super to Call Superclass Constructors

To call a constructor from its superclass:


super(parameter-list);

parameter-list is defined by the constructor in the superclass.
super(parameter-list) must be the first statement executed inside a subclass' constructor.

Here is a demo for how to use super to call constructor from parent class.

 
class Box {
  private double width;
  private double height;
  private double depth;

  Box(Box ob) { // pass object to constructor
    width = ob.width;
    height = ob.height;
    depth = ob.depth;
  }
  Box(double w, double h, double d) {
    width = w;
    height = h;
    depth = d;
  }
  double volume() {
    return width * height * depth;
  }
}
class BoxWeight extends Box {
  double weight; // weight of box
  BoxWeight(Box ob) { // pass object to constructor
    super(ob);
  }
}
public class Main {
  public static void main(String args[]) {
    Box mybox1 = new Box(10, 20, 15);
    BoxWeight myclone = new BoxWeight(mybox1);
    double vol;

    vol = mybox1.volume();
    System.out.println("Volume of mybox1 is " + vol);
  }
}

This program generates the following output:


Volume of mybox1 is 3000.0

Use super to reference members from parent class

Its general form is:


super.member

member can be either a method or an instance variable.

Let's look at the following code.

 
class Base {
  int i;
}
class SubClass extends Base {
  int i; // this i hides the i in A
  SubClass(int a, int b) {
    super.i = a; // i in A
    i = b; // i in B
  }
  void show() {
    System.out.println("i in superclass: " + super.i);
    System.out.println("i in subclass: " + i);
  }
}
public class Main {
  public static void main(String args[]) {
    SubClass subOb = new SubClass(1, 2);
    subOb.show();
  }
}

This program displays the following:


i in superclass: 1
i in subclass: 2

A Multilevel Hierarchy Example

 
class Box {
  private double width;
  private double height;
  Box(Box ob) { 
    width = ob.width;
    height = ob.height;
  }
  Box(double w, double h) {
    width = w;
    height = h;
  }
  Box() {
    width = -1; // use -1 to indicate
    height = -1; // an uninitialized
  }
  Box(double len) {
    width = height = len;
  }
  double volume() {
    return width * height;
  }
}
class BoxProduct extends Box {
  double price; // price of box
  BoxProduct(BoxProduct ob) { // pass object to constructor
    super(ob);
    price = ob.price;
  }
  BoxProduct(double w, double h, double m) {
    super(w, h); // call superclass constructor
    price = m;
  }
  BoxProduct() {
    super();
    price = -1;
  }
  BoxProduct(double len, double m) {
    super(len);
    price = m;
  }
}
class BoxSold extends BoxProduct {
  double cost;
  BoxSold(BoxSold ob) {
    super(ob);
    cost = ob.cost;
  }
  BoxSold(double w, double h, double m, double c) {
    super(w, h, m); 
    cost = c;
  }
  BoxSold() {
    super();
    cost = -1;
  }
  BoxSold(double len, double m, double c) {
    super(len, m);
    cost = c;
  }
}

public class Main {
  public static void main(String args[]) {
    BoxSold shipment1 = new BoxSold(10, 20, 10, 3.41);

    double vol = shipment1.volume();
    System.out.println("Volume of shipment1 is " + vol);
    System.out.println("Weight of shipment1 is " + shipment1.price);
    System.out.println("Shipping cost: $" + shipment1.cost);
    System.out.println();
  }
}

The output of this program is shown here:


Volume of shipment1 is 200.0
Weight of shipment1 is 10.0
Shipping cost: $3.41

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