Correct way to declare a method with two type parameters in inheritance hierarchy

Question

When writing a method that takes two objects, with two type parameters in a subtype-supertype relation, what is the best way of declaring your intentions out of these options?

1. Declare both super and extends :

    public static <T> void copy(List<? super T> dst, List<? extends T> src) { ... } 

2. Declare just the extends :

    public static <T> void copy(List<T> dst, List<? extends T> src) { ... } 

3. Declare just the super :

    public static <T> void copy(List<? super T> dst, List<T> src) { ... } 

From my understanding, all three are correct, and are equivalent to each other, as all you're interested in is the relative inheritance of the type arguments of dst and src . So which is better?

Answer 1

I think extends is most common and if there is no need to use both you shouldn't. So I'd go with extends only.

So there is not a correct way, unless you have a convention that specifies it.