Unchained Nullity

Nullity in Java can be a subtle business. If you call get(key) on a plain HashMap with a key that is not in the map, you will get null back. If you call put(key, null) on the same HashMap, then get(key), you will also get null back. In the former case, containsKey(key) will return false; in the latter (after insertion), true: the null you inserted is ge…