Actually, you can do what you want. If you want to provide multiple interfaces or a class plus interfaces, you have to have your wildcard look something like this: <T extends ClassA & InterfaceB> See the Generics Tutorial at sun.com, specifically the Bounded Type Parameters section, at the bottom of the page. You can actually … Read more
In your first question, <? extends T> and <? super T> are examples of bounded wildcards. An unbounded wildcard looks like <?>, and basically means <? extends Object>. It loosely means the generic can be any type. A bounded wildcard (<? extends T> or <? super T>) places a restriction on the type by saying … Read more
tl;dr: “PECS” is from the collection’s point of view. If you are only pulling items from a generic collection, it is a producer and you should use extends; if you are only stuffing items in, it is a consumer and you should use super. If you do both with the same collection, you shouldn’t use … Read more