How to cast List<? extends Foo> to List<Foo>

Question

Spring Batch's ItemWriter interface is this:

write(List<? extends T> items); 

I'd like the ItemWriter to call a Service but my service has this:

process(List<T> items); 

AFAIK, Java Generics are strict about casting types within collections.

Answer 1

List<? extends Foo> list1 = ...
List<Foo> list2 = Collections.unmodifiableList(list1);

Reason why list2 has to be read-only view of list1 is nicely explained in an answer of Generics : List is same as List?

Answer 2

Just go ahead and cast it. For reading, List<? extends Foo> is certainly a List<Foo>, the cast is absolutely safe. You can wrap it with Collections.unmodifiableList() if you are paranoid.

List<? extends Foo> foos1 = ...;

@SuppressWarnings("unchecked")
List<Foo> foos2 = (List<Foo>)(List<?>)foos1;