Need Explanation for a question in Generics

[thanvi janu]

Hi All,

  public class AnimalHouse<E> {
    private E animal;
    public void setAnimal(E x) {
        animal = x;
    }
    public E getAnimal() {
        return animal;
    }
}

public class Animal{
}

public class Cat extends Animal {
}

public class Dog extends Animal {
}
  

Question:

    
AnimalHouse<?> house = new AnimalHouse<Cat>();
house.setAnimal(new Cat());

Answer was: Fails to compile.

can some one explain me the answer.


Thanks a lot,
thanvi

[Rob Spoor]

Because you use a wildcard in the declaration of house, the compiler no longer knows it is a AnimalHouse<Cat>. For all it knows it could be an AnimalHouse<Dog>, AnimalHouse<Bird> or even AnimalHouse<String>*. You need to replace the "?" with either "Cat" or "? super Cat".


* Change the declaration of AnimalHouse to this:

public class AnimalHouse<E extends Animal> {

This way the generic type is bound to be Animal or any subtype.