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贬低或抛弃多态性——较小的邪恶是什么?

downcast polymorphism java

2

meow · 技术社区 · 8 年前

假设我们有一个基类消息:

public abstract Class Message {

    Object content;

    public Message(Object content) {
        this.content = content;
    }
}

public Class Packet extends Message {

    public Packet(Long largeNumber) {
        super(largeNumber);
    }

    public Long unpack() {
        return (Long) content;
    }
}

public Class Letter extends Message {

    public Letter(Short smallNumber) {
        super(smallNumber);
    }

    public Short unpack() {
        return (Short) content;
    }
}

现在假设我们有一个sender类,它将消息发送到某处。

public Class Sender {

    public send(Message msg) {
        // send it somewhere
    }
}

public Class Receiver {

    receive(Message msg) {
       // do something with the msg 
   }
}

然而,receiver类只是获取超类消息,并且事先不知道它将接收哪个子类。那么我现在该如何“解包”信息呢?

Packet packet = (Packet) msg;

但不知何故,这让人感觉不对,因为它从一开始就忽视了多态性这一点。只发送绝对子消息是否更好? 或者,有没有解决我看不到的问题的方法(例如,在某些变体中使用泛型-我对它们不太熟悉)?

4 回复 | 直到 8 年前

1

3

Sergey Kalinichenko 8 年前

解包消息可以由消息本身使用 visitor pattern :

public abstract Class Message {
    void send(Receiver r) {
        r.receive(this); // Catch-all
    }
}

public Class Packet extends Message {
    void send(Receiver r) {
        r.receive(this); // Overload for packets
    }
}

public Class Letter extends Message {
    void send(Receiver r) {
        r.receive(this); // Overload for letters
    }
}

public Class Receiver {
    // There is an overload for each subclass
    receive(Packet packet) {
    }
    receive(Letter letter) {
    }
    // This is the catch-all implementation
    receive(Message msg) {
    }
}

2

1

Mordechai 8 年前

要走的路是泛型:

public abstract class Message<T> {

    private T content;

    public T unpack() {
        return content;
    }

}

3

0

ÐÐ¸Ð¼Ð° ÐÐ¾Ð´Ð¸ÐºÐ¾Ð² 8 年前

    public abstract class Message {
     public abstract String unpack();
     // other code
}
public class Letter extends Message {
    public String unpack() {
        // your unpacked code
    }
}
public class Receiver{
    receive(Message msg){
        String s = msg.unpack(); 
        /* you can invoke unpack method because earlier declared it as abstract in parent class */
    }
}

4

0

OutOfNPE 8 年前

我建议为每种类型的消息创建处理程序的映射。消息的内容不应丢失其类型。信息和信件示例:

abstract class Message<T> {
    private T content;
    protected Message(T content) {
        this.content = content;
    }
    public T getContent() {
        return content;
    }
}

class Letter extends Message<Short> {
    public Letter(Short number) {
        super(number);
    }
}

interface MessageHandler<T extends Message> {
    void handle(T message);
}

class MessageHandlersProvider {
    Map<Class<? extends Message>, MessageHandler<? extends Message>> handlers = new HashMap<>();
    <T extends Message> void addHandler(Class<T> messageType, MessageHandler<T> handler) {
        handlers.put(messageType, handler);
    }

    MessageHandler<?> getHandler(Class<? extends Message> messageType) {
        return handlers.get(messageType);
    }
}

class LetterHandler implements MessageHandler<Letter> {
    @Override
    public void handle(Letter letter) {
        //Short number = letter.getContent();
        //do anything
    }
}

现在,您可以创建提供者,并通过处理程序按以下方式填充它:

MessageHandlersProvider handlersProvider = new MessageHandlersProvider();
handlersProvider.addHandler(Letter.class, new LetterHandler());

然后获取处理程序并处理任何类型的消息:

MessageHandler<?> handler = handlersProvider.getHandler(message.getClass());
handler.handle(message);