HashSet delegates all its functions to HashMap

This is an old one.  In Java, HashSet delegates all its functions to HashMap.

What goes on under the covers of a HashSet<T> is:

• A member variable, map, of type HashMap<T, Object> is maintained.
• All operations on HashSet is delegated to a corresponding operation on HashMap - add(), remove(), contains(), etc.
• A dummy object

add(T objOfTypeT)
Invokes put(objOfTypeT, dummy object)

remove(T objOfTypeT)
Invokes HashMap's remove(objOfTypeT)

size()
returns map's size

contains(T objOfTypeT)
returns map.containsKey(objOfTypeT)

iterator()
returns map's keySet's iterator