Java 泛型 - 遇到棘手的情况时遇到麻烦

Question

我已经在这个问题上苦苦挣扎了好几个小时了，我已经把它提炼成一小段代码。你能看出错误在哪里吗？

public class Experiment {
   class BaseMessage<Messageable> {
      Messageable once;

      BaseMessage(Messageable x) {
         once = x;
      }
   }

   class BaseEntity {
      boolean handleMessage(BaseMessage<BaseEntity> theMessage) {
         return false;
      }

   }

   class StateMachine<Thing> {
      public boolean handleMessage(BaseMessage<Thing> msg) {
         return true;
      }

   }

   class VehicleEntity extends BaseEntity {
      public StateMachine<VehicleEntity> state;

      @Override
      boolean handleMessage(BaseMessage<VehicleEntity> theMessage) {
         return state.handleMessage(theMessage);
      }
   }
}

我在这一行收到编译器错误:

boolean handleMessage(BaseMessage<VehicleEntity> theMessage) {

错误是:"The method handleMessage(Experiment.BaseMessage<Experiment.VehicleEntity>) of type Experiment.VehicleEntity must override or implement a super type method"

请注意，这是我的真实代码的示例子集，旨在将其减少到最小值，因此代码不会执行任何明智的操作。哇，如果你能弄清楚这一点，我将非常感激!

Answer 1

正如《悲惨变量》所述，在VehicleEntity中你有handleMessage(BaseMessage<VehicleEntity> theMessage)不是handleMessage(BaseMessage<BaseEntity> theMessage)并且编译器期望您的原始代码中使用后者。

主要的替代方案是 BaseEntity 采用“self”类型参数，如下所示:

class BaseEntity<T extends BaseEntity<T>> {
  boolean handleMessage(BaseMessage<T> message) {
    ...
  }
}
class VehicleEntity extends BaseEntity<VehicleEntity> {
  boolean handleMessage(BaseMessage<VehicleEntity> message) {
    ...
  }
}