How Hashset avoids duplicates

Question

HashSet internally calling HashMap to avoid duplicates in the implementation

  public HashSet() {
    map = new HashMap<E,Object>();
    }

public boolean add(E e) { 
return map.put(e, PRESENT)==null;
}

For Example

Code:

Set hashSet = new HashSet();
hashSet.add("Abraham");
hashSet.add("Billy");       
hashSet.add("Billy");       
System.out.println("HashSet Value " +hashSet.toString());

Output:

HashSet Value [Billy, Abraham]

Answer 1

In the Map interface, each key is also unique (java docs):

An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

This means, the HashMap is already taking care of avoiding duplicate keys, which are the elements of the HashSet