How HashSet Works Internally in Java?

HashSet uses HashMap internally to store its 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 some sample code of the constructors of HashSet class.

private transient HashMap map;

//Constructor - 1

public HashSet()

{

        map = new HashMap<>();          //Creating internally backing HashMap object

}

 //Constructor - 2

public HashSet(int initialCapacity)

{

        map = new HashMap<>(initialCapacity);          //Creating internally backing HashMap object

}

You can notice that each and every constructor internally creates one new HashMap object.

How HashSet Works Internally in Java?

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 its key and constant called “PRESENT” as its 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 its 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

        set.add("RED");

        set.add("GREEN");

        set.add("BLUE");

         set.add("PINK");

        //Removing "RED" from HashSet

        set.remove("RED");

    }

}

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.

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.