Unique Elements : Since HashSet and TreeSet both implements Set interface . Both are allowed to store only unique elements in their objects. Thus there can never be any duplicate elements inside the HashSet and TreeSet objects.
Not Thread Safe : HashSet and TreeSet both are not synchronized or not thread safe.HashSet and TreeSet, both implementations are not synchronized. If multiple threads access a hash set/ tree set concurrently, and at least one of the threads modifies the set, it must be synchronized externally.
Clone() method copy technique: Both HashSet and TreeSet uses shallow copy technique to create a clone of their objects .
Fail-fast Iterators : The iterators returned by this class's method are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
