(generics) Understanding Joshua Bloch's java reloaded presentation

[Pho Tek]

Joshua Bloch PDF

On slide 20:

Don't confuse bounded wildcards with bounded type variables.

Bounded wildcards
void sell(Collection<? extends T> myLot);
- Major use: restrict input parameters
- Can use super

Bounded type variables
<T extends Number> T sum(List<T> x) { � }
- Restricts actual type parameter: Works for parameterized classes and methods
- Can't use super

What does he mean when he writes Can use super and Can't use super ?

My guess below; but I require your confirmation:

"Can use super" means that I can call #sell( Collection<T> c ).

"Can't use super" means that #sum( List<Number> ) is disallowed

Am I correct ?
[ January 15, 2008: Message edited by: Pho Tek ]

[Jim Yingst]

No, he's talking about whether or not you can use the actual keyword "super" as part of the code with each of those types. For example with a wildcard, you can have <?>, <? extends X>, <? super Y> or <? extends X super Y> (where X and Y are some classes or interfaces; could be anything). With a bounded type variable though, you can't use "super" at all. You can only have things like <T> or <T extends X>. That's what he's saying.

[Pho Tek]

Got it. So wildcarding means any type expression that contains a ?.

[Jim Yingst]

Yup, exactly.