Java HashSet

HashSet in Java implements the Set interface of Collections Framework. It is used to store the unique elements and it doesn't maintain any specific order of elements.

• Can store the Null values.
• Uses HashMap (implementation of hash table data structure) internally.
• Also implements Serializable and Cloneable interfaces.
• HashSet is not thread-safe. So to make it thread-safe, synchronization is needed externally.

Example:

// Java program to show the use of HashSet
import java.util.*;

class GFG 
{
    public static void main(String[] args) 
    {
        // Instantiate an object of HashSet
        HashSet<Integer> hs = new HashSet<>();

      	// Adding elements 
        hs.add(1);
        hs.add(2);
        hs.add(1);

        // Printing the Size and Element of HashSet
        System.out.println("HashSet Size: " + hs.size());
        System.out.println("Elements in HashSet: " + hs);
    }
}

HashSet Size: 2
Elements in HashSet: [1, 2]

Hierarchy of HashSet

Declaring a HashSet

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

where E is the type of elements stored in a HashSet.

Before storing an Object, HashSet checks whether there is an existing entry using hashCode() and equals() methods. In the above example, two lists are considered equal if they have the same elements in the same order. When you invoke the hashCode()  method on the two lists, they both would give the same hash since they are equal. 

Internal Working of a HashSet

All the classes of the Set interface are internally backed up by Map. HashSet uses HashMap for storing its object internally. You must be wondering that to enter a value in HashMap we need a key-value pair, but in HashSet, we are passing only one value. 

Storage in HashMap: Actually the value we insert in HashSet acts as a key to the map Object and for its value, java uses a constant variable. So in the key-value pair, all the values will be the same.

To know more about the topic refer to Internal Working of HashSet.

Constructors of HashSet class

To create a HashSet, we need to create an object of the HashSet class. The HashSet class consists of various constructors that allow the possible creation of the HashSet. The following are the constructors available in this class.

Methods in Java HashSet

Performing Various Operations on HashSet

Let’s see how to perform a few frequently used operations on the HashSet.

1. Adding Elements in HashSet

To add an element to the HashSet, we can use the add() method. However, the insertion order is not retained in the HashSet. We need to keep a note that duplicate elements are not allowed and all duplicate elements are ignored.

Example:

// Java program to Adding Elements to HashSet
import java.util.*;

class GFG 
{
    public static void main(String[] args)
    {  
        // Creating an empty HashSet of string entities
        HashSet<String> hs = new HashSet<String>();

        // Adding elements using add() method
        hs.add("Geek");
        hs.add("For");
        hs.add("Geeks");

        // Printing all string entries inside the Set
        System.out.println("HashSet : " + hs);
    }
}

HashSet : [Geek, For, Geeks]

2. Removing Elements in HashSet

The values can be removed from the HashSet using the remove() method.

Example:

// Removing Elements in HashSet
import java.util.*;

class GFG 
{
    public static void main(String[] args)
    {
      
        HashSet<String> hs = new HashSet<String>();

        // Adding elements to above Set
        // using add() method
        hs.add("Geek");
        hs.add("For");
        hs.add("Geeks");
        hs.add("A");
        hs.add("B");
        hs.add("Z");

        // Printing the elements of HashSet elements
        System.out.println("HashSet : " + hs);

        // Removing the element B
        hs.remove("B");

        // Printing the updated HashSet elements
        System.out.println("HashSet after removing element : " + hs);

        // Returns false if the element is not present
        System.out.println("B exists in Set : " + hs.remove("B"));
    }
}

HashSet : [A, B, Geek, For, Geeks, Z]
HashSet after removing element [A, Geek, For, Geeks, Z]
B exists in Set : false

3. Iterating through the HashSet

Iterate through the elements of HashSet using the iterator() method. Also, the most famous one is to use the enhanced for loop. 

Example:

// Iterating through the HashSet
import java.util.HashSet;
import java.util.Iterator;

public class GFG 
{
    public static void main(String[] args) 
    {
      
        // Create a HashSet of Strings
        HashSet<String> hs = new HashSet<>();

        // Add elements to the HashSet
        hs.add("A");
        hs.add("B");
        hs.add("Geeks");
        hs.add("For");
        hs.add("Geeks");
        hs.add("Z");

        // Using iterator() method to iterate
      	// Over the HashSet
        System.out.print("Using iterator : ");
        Iterator<String> iterator = hs.iterator();
      
      	// Traversing HashSet
        while (iterator.hasNext())
            System.out.print(iterator.next() + ", ");

        System.out.println();

        // Using enhanced for loop to iterate
      	// Over the HashSet
        System.out.print("Using enhanced for loop : ");
        for (String element : hs)
            System.out.print(element + " , ");
      
    }
}

Using iterator : A, B, Geeks, For, Z, 
Using enhanced for loop : A , B , Geeks , For , Z , 

DSA Problems On HashSet

1. Union of Two Arrays
2. Intersection of Two Sorted Arrays
3. Distinct Elements in an Array
4. Check for Disjoint Arrays or Sets
5. Check if an Array is Subset of Other
6. Duplicate within K Distance in an Array
7. 2 Sum - Check for Pair with Target Sum
8. Only Repeating Element From 1 To n-1
9. Longest Consecutive Sequence
10. Smallest Subarray with k Distinct Numbers

Performance of HashSet

HashSet extends Abstract Set<E> class and implements Set<E>, Cloneable, and Serializable interfaces where E is the type of elements maintained by this set. The directly known subclass of HashSet is LinkedHashSet.

Now for the maintenance of constant time performance, iterating over HashSet requires time proportional to the sum of the HashSet instance's size (the number of elements) plus the "capacity" of the backing HashMap instance (the number of buckets). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important. 

• Initial Capacity: The initial capacity means the number of buckets when the hashtable (HashMap internally uses hashtable data structure) is created. The number of buckets will be automatically increased if the current size gets full. 
   
• Load Factor: The load factor is a measure of how full the HashSet is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.
   

                                     Number of stored elements in the table
Load Factor =   -----------------------------------------
                                             Size of the hash table 

Example: If internal capacity is 16 and the load factor is 0.75 then the number of buckets will automatically get increased when the table has 12 elements in it.

Effect on Performance

Load factor and initial capacity are two main factors that affect the performance of HashSet operations. A load factor of 0.75 provides very effective performance with respect to time and space complexity. If we increase the load factor value more than that then memory overhead will be reduced (because it will decrease internal rebuilding operation) but, it will affect the add and search operation in the hashtable. To reduce the rehashing operation we should choose initial capacity wisely. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operation will ever occur.

Note: The implementation in a HashSet is not synchronized, in the sense that if multiple threads access a hash set concurrently, and at least one of the threads modifies the set, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the set. If no such object exists, the set should be "wrapped" using the Collections.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the set as shown below:

Set s = Collections.synchronizedSet(new HashSet(...));

Inherited Methods

Video - HashSet in Java

Visit Course