Java泛型继承神秘错误

Question

我有一个接收器类将消息放入消息队列中，如下所示

public class Receiver<T extends MessageBase> {

  private LinkedBlockingQueue<T> messageQueue = new LinkedBlockingQueue<T>();

  public void add(T msg) {
    messageQueue.add(msg);
  }

  public void Class<T> getType() {
    ...
  }
}

尝试添加消息时

public <T extends MessageBase> void processMessage(T msg) {
    for (Receiver<? extends MessageBase> r : receivers) {
        if (msg.getClass().equals(r.getType())) {
            r.add(msg);
        }
    }
}

我得到一个

The method add(capture#3-of ? extends MessageBase) in the type 
Receiver<capture#3-of ? extends MessageBase> is not applicable 
for the arguments (T)

因为 Receiver 类被强制转换为扩展 MessageBase 是否应该添加作为 MessageBase 子类的消息？

Answer 1

当您将泛型与下界(？扩展类型语法)一起使用时，您无法将任何内容添加到您的容器中。

for (Receiver<? extends MessageBase> r : receivers) {
        if (msg.getClass().equals(r.getType())) {
            r.add(msg);// This is wrong, as you cannot add anything into Receiver
        }
    }

请参阅此答案以获取 Why it isn't allowed