Home » Java » Core Java » HashSet In Java

About Shubhra Srivastava

Shubhra Srivastava
Shubhra is a software professional and founder of ProgrammerGirl. She has a great experience with Java/J2EE technologies and frameworks. She loves the amalgam of programming and coffee :)

HashSet In Java


HashSet in Java implements Set interface i.e. it doesn’t allow duplicates. It is internally backed by a HashMap which works on the principle of hashing. 

We can store a null value in a HashSet. Its default capacity is 16 with a load factor of 0.75, where:

Load factor = Number of Stored Elements / capacity

A Java HashSet is non-synchronized. Also, there’s no guarantee to retain the insertion order of elements.

In this tutorial, we’ll learn how to work with a Java HashSet.

Instantiating HashSet:

We can create a Java HashSet using one of the following constructors:

HashSet() // default capacity of 16 with a load factor of 0.75
HashSet(int initialCapacity)
HashSet(int initialCapacity, float loadFactor)
HashSet(Collection c)

Each of these constructor usages is pretty intuitive.

Let’s quickly create a HashSet using the default constructor:

Set<Integer> set = new HashSet<>();

Commonly Used Methods:

Let’s now look at some methods that can help us manipulate over a Java HashSet:

1. boolean add(E e):

It simply adds an element to the given set, if not present already. If the element is already present, add() simply returns false:

System.out.println(set.add(1));  //true
System.out.println(set.add(2));  //true
System.out.println(set.add(3));  //true
System.out.println(set.add(1));  //false - as already present
//Note that the order of elements isn't guaranteed
System.out.println(set); //[1, 2, 3]

2. boolean contains(Object obj):

The contains() method returns true if the element exists in the referenced set, false otherwise:

System.out.println(set.contains(1)); //true
System.out.println(set.contains(4)); //false

3. boolean remove(Object obj):

As the name suggests, it removes the element obj if it exists and returns true. If no such element exists, it simply returns false:

System.out.println(set.remove(1)); //true
System.out.println(set.remove(4)); //false

Note that the HashSet also inherits removeAll() and removeIf() methods, which can be used to remove values.

4. boolean isEmpty():

It returns true for an empty set, false otherwise:

System.out.println(set.isEmpty()); // false

5. int size():

It simply returns the number of elements present in the given set.

6. void clear():

The clear() method removes all values present in the referenced set, thereby making it an empty set.

Internal Implementation:

A HashSet internally uses a HashMap to store its elements. The elements stored in a HashSet are mapped as the keys in a HashMap. The value fields of all these entries contain a constant PRESENT:

private static final Object PRESENT = new Object();

which is a dummy object.

Iterating Over HashSet:

We can use one of the following ways to iterate over the elements in a HashSet:

1. forEach():

Java 8 onwards, we can use forEach() to iterate over any Java Collection:

set.forEach(e -> System.out.println(e));

2. forEachRemaining():

Java 8 also supports forEachRemaining() construct to be used with any iterator over a Collection:

Iterator<Integer> itr = set.iterator();
itr.forEachRemaining(e -> System.out.println(e));

3. Iterate Using Iterator:

In case we’re on a Java 7 or lower versions, we can simply iterate through using an iterator:

Iterator<Integer> itr = set.iterator();
while(itr.hasNext()) {

4. Extended for Loop:

We can also use an extended for loop to traverse through the elements:

for(Integer e : set) {


In this tutorial, we learned how to create and work with a Java HashSet. We also know that the Java HashSet internally uses a HashMap for its implementation.

Be the First to comment.

Published on Java Code Geeks with permission by Shubhra Srivastava, partner at our JCG program. See the original article here: HashSet In Java

Opinions expressed by Java Code Geeks contributors are their own.

(0 rating, 0 votes)
You need to be a registered member to rate this.
Start the discussion Views Tweet it!
Do you want to know how to develop your skillset to become a Java Rockstar?
Subscribe to our newsletter to start Rocking right now!
To get you started we give you our best selling eBooks for FREE!
1. JPA Mini Book
2. JVM Troubleshooting Guide
3. JUnit Tutorial for Unit Testing
4. Java Annotations Tutorial
5. Java Interview Questions
6. Spring Interview Questions
7. Android UI Design
and many more ....
I agree to the Terms and Privacy Policy
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments