用Java进行多态调度 [英] Polymorphically Dispatching in Java

问题描述

在下文中，我希望EventHandler以一种方式处理EventA，以另一种方式处理EventB，以及另一种方式处理任何其他事件（EventC，EventD）。 EventReceiver仅接收对Event的引用并调用EventHandler.handle（）。当然，总是被调用的版本是EventHandler.handle（事件事件）。

In the following, I want EventHandler to handle EventA one way, EventB another way, and any other Events (EventC, EventD) yet another way. EventReceiver receives only a reference to an Event and calls EventHandler.handle(). The version that always gets called, of course, is EventHandler.handle(Event event).

不使用instanceOf，是否有一种多态分派的方法（可能通过另一种方法）在EventHandler或泛型中）到适当的句柄方法？

Without using instanceOf, is there a way to polymorphically dispatch (perhaps via another method in EventHandler or generics) to the appropriate handle method?

class EventA extends Event {
}

class EventB extends Event {
}

class EventC extends Event {
}

class EventD extends Event {
}

class EventHandler {
    void handle(EventA event) {
       System.out.println("Handling EventA");
    }

    void handle(EventB event) {
       System.out.println("Handling EventB");
    }

    void handle(Event event) {
       System.out.println("Handling Event");
    }
}

class EventReceiver {
    private EventHandler handler;

    void receive(Event event) {
        handler.handle(event);
    }
}    

推荐答案

声音类似于应用访客模式（的变体）的情况。 （在主流OO语言中，例如C ++，C＃和Java，方法是单一调度，即一次只能在一种类型上进行多态。访问者允许实现双重调度。）

Sounds like a case for applying (a variant of) the Visitor pattern. (In mainstream OO languages such as C++, C# and Java, methods are single dispatch, i.e. can only be polymorphic on one type at a time. Visitor allows one to implement double dispatch.)

然而，这要求您能够修改事件类，并创建依赖关系从事件到（基本接口） EventHandler 。

This however requires that you be able to modify the Event classes as well, and creates a dependency from Events to (a base interface of) EventHandler.

class EventA extends Event {
  public handleBy(EventHandler eh) {
    eh.handleEventA(this);
  }
}

class EventB extends Event {
  public handleBy(EventHandler eh) {
    eh.handleEventB(this);
  }
}

class EventHandler {
    void handleEventA(EventA event) {
       System.out.println("Handling EventA");
    }

    void handleEventB(EventB event) {
       System.out.println("Handling EventB");
    }

    void handle(Event event) {
       event.handleBy(this);
    }
}

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