如何设计一个类层次结构，该类层次结构提供有关子类的信息而无需实例化它

Question

I'm trying to design classes for spells in a game.

First i have a player with a spelbook, which indicates which spells are available for this player

public class Player {
    private List<Class<? extends Spell>> speellBook;
}

now i created some spells

public abstact class Spell { 
    public abstact double getDmg(); 
    private double position x;
    private double position y;
}
public class FireBolt extends Spell {
    public double getDmg() { return 15; }
}
public class FrostBolt extends Spell {
    public double getDmg() { return 10; }
}

Now i want to show a tooltip about the FireBolt, which shows that this spell does 15 damage whithout instantiating it. It should only instantiated when the player really casts it.

I already thought of a static field but static fields cannot be overidden by the real spells.

Then i thought of an singleton pattern, but since i store the current position in my spells, too, this won't work either

Is there any standard pattern for this? I only found the prototype-pattern but sice i need a singleton it won't be inheritable

Answer 1

You're trying to apply the inheritance (uniformly call getDmg method on different spells) for classes, avoiding objects instantiation. As you already noticed, these requires static fields and a static method, but they do not work with inheritance. I've seen these many times and it does not work in that way.

I suggest creating template objects and use them for displaying tooltip. Example:

public class SpellTooltip {

    private Map<Class<? extends Spell>, Spell> templates = new HashMap<>();

    {
        templates.put(Firebolt.class, new Firebolt());
        templates.put(Frostbolt.class, new Frostbolt());
    }

    public int getSpellDamage(Class<? extends Spell> clazz) {
        return templates.get(clazz).getDmg();
    }
}

The implementation can vary based on the complexity of your design. Using it as-is, you should always remember about adding new types to the map.