How are HashMap and Set related in Java?

HashMap and Set in Java are connected through the Map interface.

The HashSet class internally uses a HashMap to store its elements. Each element of the Set is stored as a key in the HashMap, and a dummy object (new Object()) is used as the value.

This provides the following properties of Set:

• No duplicates, since keys in HashMap are unique.
• Fast checks for the presence of an element (contains), removal (remove), and addition (add) thanks to hashing (average complexity O(1)).

Example:

// Creating a HashSet
java.util.Set<String> mySet = new java.util.HashSet<>();

// Internal representation in HashSet:
// HashMap<String, Object> internalMap = new HashMap<>();
// when adding an element e: internalMap.put(e, PRESENT); where PRESENT is a dummy object

Thus, Set is an abstraction that is implemented using Map to ensure element uniqueness and efficient operations.