List> vs List<? extends Map>

Question

Is there any difference between

List>

and

List>
<         


        

Answer 1

Today, I have used this feature, so here's my very fresh real-life example. (I have changed class and method names to generic ones so they won't distract from the actual point.)

I have a method that's meant to accept a Set of A objects that I originally wrote with this signature:

void myMethod(Set set)

But it want to actually call it with Sets of subclasses of A. But this is not allowed! (The reason for that is, myMethod could add objects to set that are of type A, but not of the subtype that set's objects are declared to be at the caller's site. So this could break the type system if it were possible.)

Now here come generics to the rescue, because it works as intended if I use this method signature instead:

 void myMethod(Set set)

or shorter, if you don't need to use the actual type in the method body:

void myMethod(Set set)

This way, set's type becomes a collection of objects of the actual subtype of A, so it becomes possible to use this with subclasses without endangering the type system.