Removing null references from a HashSet

Since a Set can not contain the same value twice (including null, if it is supported by the specific Set implementation), simply doing set.remove(null) would be sufficient.

Note that you don't even need to check for the existence of null before, because remove(null) will simply do nothing if the Set doesn't contain null.

A HashSet, being a set, only contains one "copy" of any object, which also means that it can only contain one instance of null. Thus, you can just use HashSet.remove(null).