Can I have different copies of a static variable for each different type of inheriting class

Question

I want to have the same static variable with a different value depending on the type of class.

So I would have

public class Entity
{
     public static Bitmap sprite;

     public void draw(Canvas canvas, int x, int y)
     {
          canvas.drawBitmap(sprite, x, y, null);
     }
}

public class Marine extends Entity
{

}

public class Genestealer extends Entity
{

}

And then in my main program go:

Marine.sprite = // Load sprite for all instances of Marine
Genestealer.sprite = // Load sprite for all instances of Genestealer

I don't want to store the same sprite in every instance of the class. I want one for each type of class. I want to inherit the static sprite variable and the draw function which will draw the sprite. But I don't want the Genstealer sprite to override the Marine sprite.

Is this possible?

How would I do it?

Answer 1

So make one Sprite and give everyone instance variables. They are just references; hardly more than pointers.

Answer 2

Use an abstract method:

public class Entity
{
     public abstract Bitmap getSprite();

     public void draw(Canvas canvas, int x, int y)
     {
          canvas.drawBitmap(getSprite(), x, y, null);
     }
}

public class Marine extends Entity
{
    public Bitmap getSprite() {
        return /*the sprite*/;
    }
}

The sprite returned by getSprite can be a static if you like. Nice things about this approach:

• You can't (easily) forget to include a sprite in your subclass, since the compiler will complain if you don't implement the abstract method.

• It's flexible. Suppose a Marine should look different once he "levels up". Just change Marine's getSprite method to take the level into account.

• It's the standard OO-idiom for this sort of thing, so people looking at their code won't be left scratching their heads.

Answer 3

It's not possible for it to be static in the super class. That static variable in the super class is shared (effectively a singleton) by all the sub classes. Anytime you change that (say, by instantiating a sub class), that change is reflected in all the other sub class instances.

Instead, make the sprite static in the subclasses, and then use the method structure described by Laurence.