A Superclass Variable Can Reference a Subclass Object : Access Control « Class Definition « Java Tutorial

1. Java Tutorial
2. Class Definition
3. Access Control

class RefDemo {
  public static void main(String args[]) {
    BoxWeight weightbox = new BoxWeight(3, 5, 7, 8.37);
    Box plainbox = new Box();
    double vol;

    vol = weightbox.volume();
    System.out.println("Volume of weightbox is " + vol);
    System.out.println("Weight of weightbox is " + weightbox.weight);
    System.out.println();

    plainbox = weightbox;

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

  }
}

class Box {
  private double width;

  private double height;

  private double depth;

  Box(Box ob) { 
    width = ob.width;
    height = ob.height;
    depth = ob.depth;
  }

  Box(double w, double h, double d) {
    width = w;
    height = h;
    depth = d;
  }

  Box() {
    width = -1; 
    height = -1;
    depth = -1; 
  }

  Box(double len) {
    width = height = depth = len;
  }

  double volume() {
    return width * height * depth;
  }
}

class BoxWeight extends Box {
  double weight;

  BoxWeight(BoxWeight ob) { 
    super(ob);
    weight = ob.weight;
  }

  BoxWeight(double w, double h, double d, double m) {
    super(w, h, d); 
    weight = m;
  }

  BoxWeight() {
    super();
    weight = -1;
  }

  BoxWeight(double len, double m) {
    super(len);
    weight = m;
  }
}