Java Generics WildCard: <? extends Number> vs <T extends Number>

There is no difference in this case, because T is never used again.

The reason for declaring a T is so that you can refer to it again, thus binding two parameter types, or a return type together.

The difference is you can't refer to T when using a wildcard.

You aren't right now, so there is "no difference", but here's how you could use T to make a difference:

static <T extends Number> T getElement(List<T> l) {
    for (T t : l) {
        if (some condition)
            return t;
    }
    return null;
}

This will return the same type as whatever is passed in. eg these will both compile:

Integer x = getElement(integerList);
Float y = getElement(floatList);

T is a bounded type, i.e. whatever type you use, you have to stick to that particular type which extends Number, e.g. if you pass a Double type to a list, you cannot pass it a Short type as T is of type Double and the list is already bounded by that type. In contrast, if you use ? (wildcard), you can use "any" type that extends Number (add both Short and Double to that list).