Question

I have the following problem. Am trying to make a polymorphic call and the result would depend on the variable that changes value depending on the underlying class. Tried different things however it doesn’t work. Please let me know what should be changed. Problem is that although c.w reads both the local variable w, which is defaulted to 0 and reads the one from appropriate class it always defaults to 0. Here is the code:

class Cycle{
    private int w = 0;
    public void move(){
        System.out.println("Cycle moving");
    }
    public int wheels(Cycle c){
        switch (c.w){

            case 1:     return 1; 
            case 2:     return 2;
            case 3:     return 3;
            default:    return 0;
        }
    }
}
class Unicycle extends Cycle{
    public int w = 1;
    public void go(){
        System.out.println("Unicycle go");
    }
}
class Bicycle extends Cycle{
    public int w = 2;
    public void go(){
        System.out.println("Bicycle go");
    }
}
class Tricycle extends Cycle{
    public int w = 3;
    public void go(){
        System.out.println("Tricycle go");
    }
}

public class TestCycle {

    public static void ride(Cycle c){
        c.move();
        int now = c.wheels(c);
        System.out.println(now);

    }

    public static void main(String[] args){
        Bicycle b = new Bicycle();
        ride(b);
        Unicycle u = new Unicycle();
        ride(u);
        Tricycle t = new Tricycle();
        ride(t);
    }

}

Answer 1

Your problem (well one of them) is that you are redefining the class variable ‘w’ in each of your subclasses. Define it one as a member of ‘Cycle’ and have each subclass set it correctly in their constructors.

class Cycle{
    protected int w;
    public void move(){
        System.out.println("Cycle moving");
    }
    public int wheels(){
        return w;
    }
}

class Unicycle extends Cycle{
    public Unicycle() {
        w = 1;
    }
     public void go(){
        System.out.println("Unicycle go");
    }
}

Or you can define an abstract method called ‘wheels()’ in the superclass and override it in the subclasses. It’s a matter of taste.