在这种情况下，有没有办法避免if-else和动态转换？

Question

For example, for a game I have some Skill, which is data object:

public interface Skill{
public String getName();
}

public class Attack implements Skill{
    public String getName(){ return "Attack"; }
    public int power;
}

public class Speak implements Skill{
    public String getName(){ return "Speak"; }
    public String speech;
}

---

To apply the skills during the game, I need some SkillHandler for each corresponding skill:

public interface SkillHandler{
    public void apply(Skill skill);
}

public class AttackHandler{
    @Override
    public void apply(Skill skill){
        Attack attack=(Attack)skill;
        Player player=Global.getPlayer();
        Enemy enemy=Global.getEnemy();
        enemy.hp=enemy.hp-attack.power;
        //some other code for follow up handle
    }
}

public class SpeakHandler{
    @Override
    public void apply(Skill skill){
        Speak speak=(Speak)skill;
        Label label=new Label(speech);
        this.displayOnTop(label);
    }
}

I use one SkillHandler for each Skill because I don't want the Skill depend on SkillHandler, and PlayerAttackStateHandler would apply each skill:

public class PlayerAttackStateHandler{
    public PlayerAttackHandler(){
        Skill[] skills=Global.getSkills();
        for(int i=0;i<skills.length;i++){
            SkillHandler skillHandler=null;

            if(skills[i].getName().equals("Attack")){
                skillHandler=new AttackHandler();
            }else if(skills[i].getName().equals("Speak")){
                skillHandler=new SpeakHandler();
            }

            skillHandler.apply(skills[i]);
        }
    }
}

I know this design is ill-formed because it has at least 2 problems:

1. I need to update the long if-else chain if a new Skill as well as new SkillHandler is added, which seems does't follow open-closed principle

2. it has a dynamic cast in each SkillHandler

My question is, is there any design pattern to eliminate both if-else and dynamic cast (if possible) in this case, while keep the Skill not depend on SkillHandler?

Answer 1

It seems that your implementation has lot in common with visitor pattern, something like this:

public interface ISkillable //this is your Skill
{
    public int GetPower();
    public string GetSpeak();
}

public interface IVisitable //player or npc
{
    public void Accept(IVisitor visitor)
}

public interface IVisitor //AttackHandler or SpeakHandler
{
    public void ApplySkill(ISkillable skillable)
}

public class Player implements ISkillable, IVisitable
{
    ...
    public void Accept(IVisitor visitor)
    { 
        visitor.Visit(this); 
    }
}

public class AttackVisitor implements IVisitor
{
    public void Visit(ISkillable skillable)
    {
        //do something with power
    }
}

and then example how it can be used is

player.Accept(new AttackVisitor(/*you can provide additional info like enemy*/));
player.Accept(new SpeakVisitor());

Answer 2

See Attack and Speak as capabilities some agent could possess.

I would consider testing capabilities/features:

interface Attacking { void attack(); }
interface Speaking { void speak(); }

Animal animal = ...

Optional<Attacking> attacker = animal.lookup(Attacking.class);
attacker.ifPresent(a -> a.attack());

Optional<Speaking> speaker = animal.lookup(Speaking.class);
speaker.ifPresent(sp -> sp.speak());

Animal need not implement any interface, but you can look up (lookup or maybe as) capabilities. This is extendible in the future, dynamic: can change at run-time.

Implementation as

private Map<Class<?>, ?> map = new HashMap<>();

public <T> Optional<T> lookup(Class<T> type) {
     Object instance = map.get(type);
     if (instance == null) {
         return Optional.empty();
     }
     return Optional.of(type.cast(instance));
}

<S> void register(Class<S> type, S instance) {
    map.put(type, instance);
}

The implementation does a safe dynamic cast, as register ensures the safe filling of (key, value) entries.