Generally at least in Java or similar languages it's better to compose rather than implement or extend, i.e. HashMap works fine for most cases, and you may not

Generally at least in Java or similar languages it's better to compose rather than implement or extend, i.e. HashMap works fine for most cases, and you may not really need to implement java.util.Map if you're building something "special" anyway. For frameworks and library authors, I can see more of an argument for it, I just generally have never subclassed collections except in certain cases such as building an LRU map which Java doesn't have.