java - java中如何实现模式匹配，避免instanceof？

Question

目前我不知道如何避免我的代码中出现代码异味。我尝试了几种模式(策略、访客)，但它们没有提供干净且可维护的解决方案。这是我的策略模式代码示例:

public interface Strategy {
  <T> T foo(FirstParam firstParam, SecondParam secondParam);
}

public class StrategyOne implements Strategy {
  FirstReturnType foo(FirstParam firstParam, SecondParam secondParam);
}

public class StrategyTwo implements Strategy {
  SecondReturnType foo(FirstParam firstParam, SecondParam secondParam);
}

@Setter
public class Context {
    private Strategy strategy;
    public void execute(FirstParam firstParam, SecondParam secondParam) {
        if (strategy != null) {
            strategy.fo(firstParam, secondParam);
        }
    }
}

还有一个对象的例子。

public abstract class Action {
 abstract void bar();
} 

public class ActionOne extends Action {
  void bar() {}
}

public class ActionTwo extends Action {
  void bar() {}
}

我想让这段代码更干净

public class ActionExecutor {
   private Context context;
   private FirstParam firstParam;
   private SecondParam secondParam;
   public ActionExecutor(FirstParam firstParam, SecondParam secondParam) {
    this.context = new Context();
    this.firstParam = firstParam;
    this.secondParam = secondParam;
   }

  public void doSmth(Item item) {
    Action action = item.getAction();
    if(action instanceof ActionOne) {
     context.setStrategy(new StrategyOne());
    }
    if(action instanceof ActionTwo) {
     context.setStrategy(new StrategyTwo());
    }
    context.execute(firstParam, secondParam);
  }
}

想法是针对特定对象类型执行特定操作。但我不知道如何避免在这种情况下使用 instanceof。

Answer 1

我头顶的两种方式。

public abstract class Action {
 public Strategy strategy;
 abstract void bar();
} 

public class ActionOne extends Action {
  void bar() {}
   // set strategy here,  possibly
}

public class ActionTwo extends Action {
  void bar() {}
}

public void doSmth(Item item) {
    Action action = item.getAction();
    action.strategy.execute(firstParam, secondParam);
  }

第二种方式，在你的所有 Action 中都有一个枚举，并通过在你的抽象类构造函数中将它声明为参数来强制它。然后只需使用 switch in 而不是 instanceof