Java OOP instance of class

Question

I have following classes structure extention: Entity > Creature > (abstract)Player > Mage. In Mage class I implement interface iCastable with method castSpell(). In main method I create new Mage(...).

Problem is that when I send it as a prop of Player class someMethod(Player player), I cannot use methods implemented from interface iCastable. I can only use methods from Creature class via player.getCreaure.whaterver() because Player extend it. How can I solve that issue?

I do not want to send it as a prop of Mage class, because I want to use all my other classes like Warrior for example. I also want to avoid player instanceof Mage, because if I had 1000 classes I must do 1000 check for every method. Do you have any ideas how to solve that?

EDIT added code

public class Creature extends Entity {...}

public abstract class Player extends Creature {
    public Player(String name) {
        super(name);
    }
    public abstract void attack();
}
public class Mage extends Player implements iCastable {
    ...
    @Override
    public void castSpecial() {...}
}

public static void main(String[] args) {
   Mage mage = new Mage("Mage");
   Duel duel = new Duel(mage, monsters);
}
public class Duel {
    private Player player;
    ...
    public Duel(Player player, ArrayList<Monster> monsters) {
        this.player = player;
        ...
    }
    private void castSpecial() {
        // error here
        player.castSpecial();
    }
}

I am trying to do something like player.getInstanceClass(mage, warrior or whatever).cashSpecial()

Why do you extend the interface iCastable rather than implementing it — Luke Garrigan
– Luke Garrigan, Commented Dec 10, 2017 at 12:20
It would help if we could see code, us programmer are better at reading code than English :) — Luke Garrigan
– Luke Garrigan, Commented Dec 10, 2017 at 12:22
on what class/interface is the someMethod(Player player) defined ? — Mohammed Aouf Zouag
– Mohammed Aouf Zouag, Commented Dec 10, 2017 at 12:24
I suspect we need to understand more about the intended structure and functionality here. Based on the description alone... If the method expects a Player then the method's internal functionality shouldn't care what kind of Player. If the method's internal functionality is specific to a Mage then it should expect a Mage. What exactly does someMethod() do? If you were to give it a meaningful and descriptive name, what would it be? — David
– David, Commented Dec 10, 2017 at 12:24

Stephen C · Accepted Answer · 2017-12-10 12:54:00Z

4

Your method can be written like this:

private void castSpecial() {
    if (player instanceof iCastable) {
        ((iCastable) player).castSpecial();
    }
}

The instanceof performs a runtime check to see if the Player instance has the correct subclass. If the check succeeds then the type cast will also succeed. (If you do the type cast without a "guard", then it will throw an ClassCastException if the player doesn't have the required type.)

Obviously, if the player does not have spell casting ability, the method does nothing.

Note: the interface name iCastable is wrong from two perspectives.

A class or interface name should never start with a lowercase letter.
The name is wrong. ICastable implies that you would "cast" a Mage. In fact, the Mage is the subject, that casts the spell, not the spell that is cast. A better name might be ISpellCaster.

edited Dec 10, 2017 at 12:54

answered Dec 10, 2017 at 12:44

Stephen C

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2 Comments

IntoTheDeep Over a year ago

This is the answer I am looking for

Sean Burton Over a year ago

Bear in mind that if you're doing a lot of instanceof checks and downcasts it can be a code smell. It seems like this use-case might be better solved using composition rather than inheritance. For example each creature could contain an array of IAbility and casting a spell could be a type of ability, used by calling IAbility.activate() for example, without any need to downcast.

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