Java泛型-遇到棘手的情况

Question

I've beat my head against the wall on this one for several hours now, and I've distilled it down to a tiny bit of code. Can you see where the error is?

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);
      }
   }
}

I get a compiler error on this line:

boolean handleMessage(BaseMessage<VehicleEntity> theMessage) {

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

Note that this is a sample subset of my real code to reduce it to its minimum so the code doesn't do anything sensible. Wow, if you could figure this out I would be super grateful!

Answer 1

As stated by Miserable Variable, in VehicleEntity you have handleMessage(BaseMessage<VehicleEntity> theMessage) not handleMessage(BaseMessage<BaseEntity> theMessage) and the compiler expects the latter in your original code.

The main alternative is for BaseEntity to take a "self" type argument as follows:

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

Answer 2

BaseMessage<VehicleEntity> is not a subtype of BaseMessage<BaseEntity> . Try this

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

Answer 3

You would like for VehicleEntity to have a handleMessage method that will accept only a BaseMessage<VehicleEntity> .

I believe this is only possible in Java, at least it cannot be checked statically. But you may want to look at Curiously recurring template pattern to see if you can make it work for you.

Answer 4

Try adding generics to the class itself

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

   }

And

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

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