Internal implementation of Set/HashSet (How Set Ensures Uniqueness)

Set Implementation Internally in Java

Each and every element in the set is unique .  So that there is no duplicate element in set .

 what happens internally when you pass duplicate elements in the  add() method of the Set object , It will return false and do not add to the HashSet , as the element is already present .So far so good .

But the main problem arises that how it returns false . So here is the answer

When you open the HashSet implementation of the add() method in Java Apis that is rt.jar , you will find the following code in it

public class HashSet<E>
extends AbstractSet<E>
implements Set<E>, Cloneable, java.io.Serializable

{
    private transient HashMap<E,Object> map;
    
    // Dummy value to associate with an Object in the backing Map
    
    private static final Object PRESENT = new Object();
    
    
    
    public HashSet() {
        map = new HashMap<>();
    }
    
    // SOME CODE ,i.e Other methods in Hash Set
    
    
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }
    
    
    
    // SOME CODE ,i.e Other methods in Hash Set
}

So , we are achieving uniqueness in Set,internally in java  through HashMap . Whenever you create an object of HashSet it will create an object of HashMap as you can see in the italic lines in the above code .

The main point to notice in above code is that put (key,value) will return

1.  null , if key is unique and added to the map
2.  Old Value of the key , if key is duplicate

So , in HashSet add() method ,  we check the return value of map.put(key,value) method with null value
i.e.

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

So , if map.put(key,value) returns null ,then
map.put(e, PRESENT)==null      will return true and element is added to the HashSet .



So , if map.put(key,value) returns old value of the key ,then
map.put(e, PRESENT)==null      will return false and element is  not added to the HashSet .