How HashSet works internally in Java?

  • HashSet uses HashMap internally to store it’s objects. Whenever you create a HashSet object, one HashMap object associated with it is also created. This HashMap object is used to store the elements you enter in the HashSet. The elements you add into HashSet are stored as keys of this HashMap object. The value associated with those keys will be a constant.

  • Every constructor of HashSet class internally creates one HashMap object. You can check this in the source code of HashSet class in JDK installation directory. Below is the some sample code of the constructors of HashSet class.

    private transient HashMap < E,Object > map;
    //Constructor - 1
    public HashSet(){
        //Creating internally backing HashMap object
        map = new HashMap<>(); 

    //Constructor - 2
    public HashSet(Collection < ? extends E > c){
        //Creating internally backing HashMap object
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16)); 

    //Constructor - 3
    public HashSet(int initialCapacity, float loadFactor){
        //Creating internally backing HashMap object
        map = new HashMap<>(initialCapacity, loadFactor); 

    //Constructor - 4
    public HashSet(int initialCapacity){
        //Creating internally backing HashMap object
        map = new HashMap<>(initialCapacity); 
  • You can notice that each and every constructor internally creates one new HashMap object.

  • Whenever you insert an element into HashSet using add() method, it actually creates an entry in the internally backing HashMap object with element you have specified as it’s key and constant called “PRESENT” as it’s value. This “PRESENT” is defined in the HashSet class as below.

    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

Let’s have a look at add() method of HashSet class.

    public boolean add(E e){
        return map.put(e, PRESENT)==null;
  • You can notice that, add() method of HashSet class internally calls put() method of backing HashMap object by passing the element you have specified as a key and constant “PRESENT” as it’s value.

  • remove() method also works in the same manner.

    public boolean remove(Object o){
        return map.remove(o)==PRESENT;

Let’s see one example of HashSet and how it maintains HashMap internally.

public class HashSetExample{
    public static void main(String[] args){
        // Creating One HashSet object
        HashSet set = new HashSet();

        // Adding elements to HashSet

        // Removing "RED" from HashSet

See the below picture how above program works internally. You can observe that internal HashMap object contains elements of HashSet as keys and constant “PRESENT” as their value.

Internal working of HashSet in Java

In the same manner, all methods of HashSet class process internally backing HashMap object to get the desired result. If you know how HashMap works, it will be easy for you to understand how HashSet works. You go through the source code of HashSet class once, you will get a clear picture about how HashSet works internally in Java.