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1) HashMap implements the Map interface. HashSet implements the Set interface.
2) In HashSet you use only one Object to search or insert. In HashMap, you use two objects (key-value) to insert.

HashSet does not allow duplicates unlike HashMap

Both are part of Java Collection Framework.

1) HashMap is derived from Map Interface. Map is nothing but key-value pair as in HashTable. HashMap key-value ordering is random.
2) HashSet is derived from Set Interface which is derived from Collection Interface.
Any set interface do not allow duplicate elements. Hashset value ordering is random.

HashSet maintains the data in a HashMap, as HashSet implements Set interface all the keys are unique (duplicates are not allowed, with only one null value as a key). and irrespective of the key we put in to HashSet the value is the same, which is of type Object.