HashSet in Java

Let’s first start with an example of the creation of a HashSet object in Java:

import java.util.HashSet;    //import HashSet Class

public class Main {
    public static void main(String[] args) {
        // Create a new HashSet to store strings
        HashSet<String> elements = new HashSet<String>();

        // This HashSet can now be used to add elements, remove elements, etc.
    }
}

Java HashSet class implements the Set interface, backed by a hash table which is actually a HashMap instance. No guarantee is made as to the iteration order of the hash sets which means that the class does not guarantee the constant order of elements over time. This class permits the null element. The class also offers constant time performance for the basic operations like add, remove, contains, and size assuming the hash function disperses the elements properly among the buckets, which we shall see further in the article.  

Java HashSet Features

A few important features of HashSet are mentioned below:

• Implements Set Interface.
• The underlying data structure for HashSet is Hashtable.
• As it implements the Set Interface, duplicate values are not allowed.
• Objects that you insert in HashSet are not guaranteed to be inserted in the same order. Objects are inserted based on their hash code.
• NULL elements are allowed in HashSet.
• HashSet also implements Serializable and Cloneable interfaces.

Declaration of HashSet

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

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

HashSet Java Example

// Java program to illustrate the concept
// of Collection objects storage in a HashSet
import java.io.*;
import java.util.*;

class CollectionObjectStorage {
  
    public static void main(String[] args)
    {
        // Instantiate an object of HashSet
        HashSet<ArrayList> set = new HashSet<>();

        // create ArrayList list1
        ArrayList<Integer> list1 = new ArrayList<>();

        // create ArrayList list2
        ArrayList<Integer> list2 = new ArrayList<>();

        // Add elements using add method
        list1.add(1);
        list1.add(2);
        list2.add(1);
        list2.add(2);
        set.add(list1);
        set.add(list2);

        // print the set size to understand the
        // internal storage of ArrayList in Set
        System.out.println(set.size());
    }
}

1

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. 

Note: HashSet does not store duplicate items,  if you give two Objects that are equal then it stores only the first one, here it is list1.

The Hierarchy of HashSet is as follows:

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.

Implementation of HashSet in Java doc:

private transient HashMap map;

// Constructor - 1
// All the constructors are internally creating HashMap Object.
public HashSet()
{
    // Creating internally backing HashMap object
    map = new HashMap();
}

// Constructor - 2
public HashSet(int initialCapacity)
{
    // Creating internally backing HashMap object
    map = new HashMap(initialCapacity);
}

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

If we look at the add() method of the HashSet class: 

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

We can notice that add() method of the HashSet class internally calls the put() method of backing the 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. It internally calls the remove method of the Map interface. 

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

HashSet not only stores unique Objects but also a unique Collection of Objects like ArrayList<E>, LinkedList<E>, Vector<E>,..etc.

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.

1. HashSet()

This constructor is used to build an empty HashSet object in which the default initial capacity is 16 and the default load factor is 0.75. If we wish to create an empty HashSet with the name hs, then, it can be created as:

HashSet<E> hs = new HashSet<E>();

2. HashSet(int initialCapacity)

This constructor is used to build an empty HashSet object in which the initialCapacity is specified at the time of object creation. Here, the default loadFactor remains 0.75.

HashSet<E> hs = new HashSet<E>(int initialCapacity);

3. HashSet(int initialCapacity, float loadFactor)

This constructor is used to build an empty HashSet object in which the initialCapacity and loadFactor are specified at the time of object creation.

HashSet<E> hs = new HashSet<E>(int initialCapacity, float loadFactor);

4. HashSet(Collection)

This constructor is used to build a HashSet object containing all the elements from the given collection. In short, this constructor is used when any conversion is needed from any Collection object to the HashSet object. If we wish to create a HashSet with the name hs, it can be created as:

HashSet<E> hs = new HashSet<E>(Collection C);

Below is the implementation of the above topics:

// Java program to Demonstrate Working
// of HashSet Class

// Importing required classes
import java.util.*;

// Main class
// HashSetDemo
class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an empty HashSet
        HashSet<String> h = new HashSet<String>();

        // Adding elements into HashSet
        // using add() method
        h.add("India");
        h.add("Australia");
        h.add("South Africa");

        // Adding duplicate elements
        h.add("India");

        // Displaying the HashSet
        System.out.println(h);
        System.out.println("List contains India or not:"
                           + h.contains("India"));

        // Removing items from HashSet
        // using remove() method
        h.remove("Australia");
        System.out.println("List after removing Australia:"
                           + h);

        // Display message
        System.out.println("Iterating over list:");

        // Iterating over hashSet items
        Iterator<String> i = h.iterator();

        // Holds true till there is single element remaining
        while (i.hasNext())

            // Iterating over elements
            // using next() method
            System.out.println(i.next());
    }
}

[South Africa, Australia, India]
List contains India or not:true
List after removing Australia:[South Africa, India]
Iterating over list:
South Africa
India

Methods in 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

// Importing required classes
import java.io.*;
import java.util.*;

// Main class
// AddingElementsToHashSet
class GFG {

    // Method 1
    // Main driver method
    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 el=ntries inside the Set
        System.out.println("HashSet elements : " + hs);
    }
}

HashSet elements : [Geek, For, Geeks]

2. Removing Elements in HashSet

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

Example:

// Java program Illustrating Removal Of Elements of HashSet

// Importing required classes
import java.io.*;
import java.util.*;

// Main class
// RemoveElementsOfHashSet
class GFG {

    // Main driver method
    public static void main(String[] args)
    {
        // Creating an
        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("Initial HashSet " + hs);

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

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

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

Initial HashSet [A, B, Geek, For, Geeks, Z]
After removing element [A, Geek, For, Geeks, Z]
Element AC exists in the 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:

import java.util.HashSet;
import java.util.Iterator;

public class HashSetIterationExample {
    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();
        while (iterator.hasNext()) {
            System.out.print(iterator.next() + ", ");
        }

        System.out.println(); // New line for clarity

        // 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, 

Time Complexity of HashSet Operations: The underlying data structure for HashSet is hashtable. So amortize (average or usual case) time complexity for add, remove and look-up (contains method) operation of HashSet takes O(1) time.

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 (HashSet 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(…));

Methods Used with HashSet

1. Methods inherited from class java.util.AbstractSet

2. Methods inherited from class java.util.AbstractCollection

3. Methods declared in interface java.util.Collection

4. Methods declared in interface java.lang.Iterable

5. Methods declared in interface java.util.Set

FAQs in HashSet in Java

Q. What is HashSet in Java?

Answer:

HashSet is a type of class, which extends AbstractSet and implements Set interfaces.

Q. Why is HashSet used?

Answer:

HashSet is used for avoiding duplicate data and to find value with the fast method.

Q. Differences between HashSet and HashMap.

Answer:

Q. Differences between HashSet and TreeSet in Java.

Answer: