[Java/OO] Question about the design of dynamic subclass hierarchy handling.

Question

I always tend to run into the following design problem that I'm never quite sure how to best resolve. It usually starts with a hierarchy of Animals at my Circus:

Animal
  Cat
    BigCat
  Dog          
  Elephant
  ...

Now, each Animal needs to be trained, so that there is a separate method for each:

public interface Trainer {
    void train( BigCat animal );
    void train( Dog animal );
    void train( Elephant animal );
    // ...
}

The problem is that the CircusDirector doesn't give a damn. He just throws Animals to the Trainer without even looking.

public class CircusDirector {
    public void work() {
        Trainer trainer = getTrainer();
        Animal animal = getAnimal();

        // ...and he doesn't know a frog from a pony,
        // so he tries to just:
        trainer.train(animal);
    }
}

Now, the Trainer can get an additional method like

void train( Animal animal );

where he'll use instanceof to send the animal to the appropriate method, but this seems ugly and doesn't come recommended. Is there a better solution using generics?

Answer 1

What you're describing looks like it could be solved quite neatly with the Visitor pattern or, a little more precisely, double-dispatch.

Have your animals implement a Trainable interface:

interface Trainable {

    accept(Trainer trainer);

}

with an implementation like:

public Dog extends Animal implements Trainable {

  //... other dog stuff

  public accept(Trainer trainer) {
     trainer.train(this);
  }

}

And then keep the trainer the same way. All the animals will get dispatched appropriately.

Answer 2

You've essentially implemented half of the visitor pattern. You can have each of your Animal's provide an acceptTrainer(Trainer t) method which then call t.train(this);

Your Director would then call animal.acceptTrainer(trainer);

Also, I think generics info gets compiled out, so you can't do any fancy stuff relying on them. It's best to get familiar with some of the patterns.