There was a reason for the creation of the Set interface; it was designed not to allow duplicate elements. A duplicate is identified using the equals() method, implemented in the class whose objects are elements of the set.
Set - HashSet does not allow duplicates
Preferring variable type Set
As in the case of List, using type Set for the variable that holds a reference to the object of the class that implements the Set interface is a good programming practice called coding to an interface. It assures independence of the client code from any particular implementation. So, it is a good habit to write Set<Person> persons = new HashSet<>(), for example.
