according to [java spec](http://docs.oracle.com/javase/6/docs/api/java/util/HashSet.html#add(E)):

add method in HashSet implementation:
> public boolean add(E e)
> 
> Adds the specified element to this set if it is not already present.
> More formally, adds the specified element e to this set if this set
> contains no element e2 such that (e==null ? e2==null : e.equals(e2)).
> If this set already contains the element, the call leaves the set
> unchanged and returns false.