There are different ways on how to implement remove, but one way to do it is to overwrite the (key, value) pair to be removed with a special "tombstone" symbol that is different from null. Another way is to shift all elements with the same hash value as the one to be removed one index to the left, and null the final position.

There are cases where contains or get would have to iterate over the whole array, basically when all the previously added elements hash-collide, i.e., are mapped to the same start-index. But every hash-implementation has pathological cases where the access methods get slow -- but if the keys are sufficiently random, you don't expect such cases to occur in practice.