How can I have an abstract method that accepts an argument of type “my type”?

Question

Let's say I have an abstract class Animal with an abstract method

public abstract Animal mateWith(Animal mate);

the problem is, if I create subclasses Snake and Armadillo, a call like this would then be legal:

mySnake.mateWith(myArmadillo);

But I only want snakes to be able to mate with snakes. I need to be able to define something like this:

public abstract Animal_Of_My_Class mateWith(Animal_Of_My_Class mate);

Is this possible in Java?

Answer 1

Self-bounded generics to the rescue:

abstract class Animal<T extends Animal<T>> {
  abstract T mateWith(T mate);
}

then:

class Animal_Of_My_Class extends Animal<Animal_Of_My_Class> {
  Animal_Of_My_Class mateWith(Animal_Of_My_Class mate) { ... }
}

Note that you can't constrain T to be the implementing class (as in, you can't require that Animal_Of_My_Class extends Animal<Animal_Of_My_Class> rather than Animal_Of_My_Class extends Animal<Another_Animal_Of_My_Class> ).