具有多个界限的Java列表泛型

Question

I'm struggling to understand why the following code doesn't work :

public <E extends Animal & IQuadruped> void testFunction()
{
    List<E> list = new ArrayList<E>();
    Dog var = new Dog();
    list.add(var);
}

with Dog being the following :

public class Dog extends Animal implements IQuadruped
{

}

And I get a compile error on the add :

The method add(E) in the type List<E> is not applicable for the arguments (Dog)

I just want to make sure my list elements extend/implement both classes, and Dog fullfil those conditions, why is it not working ?

Thank you

Answer 1

What <E extends Animal & IQuadruped> means is "a particular type that is a subtype of both Animal and IQuadruped ", and not "any type that is a subtype of both Animal and IQuadruped "

The reason it's difficult to grasp the difference is that in everyday thinking we don't make that distinction explicit. If for example you agree to go out for lunch with someone in a restaurant, and they say "any day next week is good for me", you automatically know that means you need to turn up on the same day. And not that you can go at any time and they'll be there.

In this case there's no guarantee that the E the caller chooses will definitely be Dog , so your code won't work.

The obvious wrong solution is to specify <E extends Dog> , because it would guarantee that E is a subtype of Dog . However this is wrong for the exact same reason, E could be any subtype of Dog , let's say E is a Poodle , so when you create a List<Poodle> , you won't be able to put your new Dog() in there, because it isn't a Poodle .

The correct bound is <E super Dog> , because that means E is definitely a type that you can cast a Dog instance to. It could be Dog itself, it could be Animal or IQuadruped or even Object . But it guarantees that you can put a Dog in the list.

The name of this principle is PECS: producer extends, consumer super, and you can read about it here .

Answer 2

Generics type erasure is what you are getting. By the time the JVM executes the line where you are adding to the list, the generics is already lost. To get this to work, you would have to do something like:

public <E extends Animal & IQuadruped> void testFunction(E param) {
    List<E> list = new ArrayList<E>();
    list.add(param);
}

public void addDog() {
    testFunction(new Dog())
}