CODEWITHSUNDEEP
javapolymorphism
aretai
aretai

Reputation: 1641

polymorphic call depends on variable

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);
    }

}

Upvotes: 0

Views: 245

Answers (4)

toto2
toto2

Reputation: 5326

It's probably not the answer you are looking for, but the following works. Please give more details on what you are trying to do.

public abstract class Cycle {
   protected int nWheels;
   protected String goText;

   // no constructor.

   public void go() {
     System.out.println(goText);
   }

   public int wheels() {
     return nWheels;
  }
}
...

public class Unicycle extends Cycle {

   public Unicycle() {
      nWheels = 1;
      goText = "Unicycle go";
   }
}

Note that I made Cycle abstract because I don't want it to ever be instantiated.

EDIT:

public static int getNumberOfWheels(Cycle cycle) {
   return cycle.wheels();
}

which is obviously not very useful since a simple call to cycle.wheels() would do the same as calling this function.

I'm not sure why you want to avoid constructors. Maybe you should write the exact question you are trying to answer.

Upvotes: 1

brain
brain

Reputation: 5546

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.

Upvotes: 2

Viruzzo
Viruzzo

Reputation: 3025

Since Cycle.w is private, it's not visible from its inheritors. This means that for example Tricycle.w it's not the "same" variable, and it's not visible in Cycle (that's why you always get 0). You have to make Cycle.w at least protected, then remove w from all subclasses, and set its value in each subclass's constructor to what you want.

Upvotes: 1

hvgotcodes
hvgotcodes

Reputation: 120188

the wheels method should be more like

public int getWheelCount(){
   return this.w;
}

You invoke it on the instance itself, you don't need to pass an argument. If the current instance is a Tricycle, the method will return 3, etc...

Upvotes: 1

Related Questions