Can't add value to the Java collection with wildcard generic type

Question

Why this code does not compile (Parent is an interface)?

List list = ...
Parent p = factory.get();   // returns concrete         


        

Answer 1

It's doing that for the sake of safety. Imagine if it worked:

List childList = new ArrayList();
childList.add(new Child());

List parentList = childList;
parentList.set(0, new Parent());

Child child = childList.get(0); // No! It's not a child! Type safety is broken...

The meaning of List is "The is a list of some type which extends Parent. We don't know which type - it could be a List, a List, or a List." That makes it safe to fetch any items out of the List API and convert from T to Parent, but it's not safe to call in to the List API converting from Parent to T... because that conversion may be invalid.