Hidden Features of Java

Answer 1

How about covariant return types which have been in place since JDK 1.5? It is pretty poorly publicised, as it is an unsexy addition, but as I understand it, is absolutely necessary for generics to work.

Essentially, the compiler now allows a subclass to narrow the return type of an overridden method to be a subclass of the original method's return type. So this is allowed:

class Souper {
    Collection values() {
        ...
    }
}

class ThreadSafeSortedSub extends Souper {
    @Override
    ConcurrentSkipListSet values() {
        ...
    }
}

You can call the subclass's values method and obtain a sorted thread safe Set of Strings without having to down cast to the ConcurrentSkipListSet.