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

Question

What is the difference between these 2 functions?

static void gPrint(List<? extends Number> l) {     for (Number n : l) {         System.out.println(n);     } }  static <T extends Number> void gPrintA(List<T> l) {     for (Number n : l) {         System.out.println(n);     } } 

I see the same output.

Answer 1

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.

Answer 2

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);