Wednesday, September 20, 2017

Difference between HashMap and HashSet in Java

HashMap vs HashSet is the most frequently asked question during any core java interview and interview is not said completed until they will not cover the Collection Framework and multi-threading interview and collections are uncompleted without Covering Hash Set and HashMap.
Both HashMap and HashSet are part of collection framework which allows us to work with a collection of objects. Collection Framework has their own interfaces and implementation classes. Basically, a collection is divided as Set Interface, List, and Queue Interfaces. All these interfaces have their own property also apart from they get from a collection like Set allows Collection of objects but forbids duplicate value, List allows duplicate along with indexing.Queue works on FCFS algorithm.

First, we have one look on What HashMap and HashSet are then will go for Differences between HashSet and HashMap

What is HashSet in Java?

HashMap vs HashSet, difference between HashMap and Hashset in JavaHashSet  is implementation of Set Interface which does not allow duplicate value all the methods which are in Collection Framework are also in Set Interface by default but when we are talking about Hash set the main thing is objects which are going to be stored in HashSet must override equals() and hashCode() method so that we can check for equality and no duplicate value are stored in our set.if we have created our own objects we need to implement hashCode() and equal() in such a manner that will be able to compare objects correctly when storing in a set so that duplicate objects are not stored,if we have not override this method objects will take default implementation of this method.

public boolean add(Object o)  Method is used to add an element in a set which returns false if it’s a duplicate value in case of  HashSet otherwise returns true if added successfully.

What is HashMap?

HashMap is an implementation of Map Interface, which maps a key to value.Duplicate keys are not allowed in a map.Basically map Interface has two implementation classes HashMap and TreeMap the main difference is TreeMap maintains order of the objects but HashMap will not.HashMap allows null values and null keys.HashMap is not synchronized,but collection framework provide methods so that we can make them synchronized if multiple threads are going to access our hashmap and one thread structurally changes our map.

public Object put(Object Key,Object value) method is used to add an element in the map.

You can read more about HashMap in my article How HashMap works in Java and Difference between HashMap and hashtable in Java

Difference between HashSet and HashMap in Java

Following are some differences between HashMap and HashSet:

Hash Set
HashMap  is an implementation of Map interface
HashSet is an implementation of Set Interface
HashMap Stores data in form of  key-value pair
HashSet Store only objects
Put method is used to add element in map
Add method is used to add element is Set
In hash map hashcode value is calculated using key object
Here member object is used for calculating hashcode value which can be same for two objects so equal () method is used to check for equality if it returns false that means two objects are different.
HashMap is faster than HashSet because unique key is used to access object
HashSet is slower than Hashmap

Please let me know if you need any other difference between HashSet and HashMap in Java and I will add them here.

Related Java Tutorials

Further Reading
Java Fundamentals, Part 1 and 2
Java Programming Interview Exposed
Java Fundamentals: Collections


james said...

hashCode can be the same for 2 keys in a HashMap so HashMap uses equals.

In Oracle's Java implementation HashSet is only a wrapper around a HashMap.

Javin @ java enum examples said...

Yes you are correct, hashcode is used to find the bucket location and then while comparing object it uses equals in place of hashcode. you can read more on this on How HashMap works in Java

Anonymous said...

You made a good point, but the explanation is horrible!

Vicky said...

Another difference between HashMap vs HashSet is that HashMap uses put() method while HashSet uses add() method. HashMap requires two object as key and value , while HashSet only need one object to store. Overall HashSet vs HashMap is comparison between Map and Set Collection type in java.

nitus said...

HashSet internally uses HashMap to store Objects.Objects are stored as a key in HashMap and a new dummy Object is placed as value.Since Hashmap does not allow duplicate key,HashSet maintains its uniqueness.

nitus said...

Please share your view on Vector Vs synchronized(arrayList)

Anonymous said...

So there is no concept of key object in HashSet.
Am i right?

Anonymous said...

As we know that HashSet is the wrapper on the HashMap to implement unique set i have below 2 questions

1. why HashSet does not support get() method like the HashMap?

2. Why HashSet is slower than HashMap?

Ajay Yadav said...

What is the underlying implementation behind HASH SET & HASH MAP--- the same concept of key value pair, now how can HASH MAP is faster than HASH SET when conceptual implementation behind both collection is same?

Sergey Rogov said...

1. Because HashSet does not implement Map interface and Set interface does not declare get() method.
And there is no key, that you can pass to get() method in case of a set (since all keys are values). If you need to check that specified key exists in the set just use contains() method.
2. It should not be slower, because HashMap does not allow duplicated keys as well. So, performance in worst case (when we need to call to hashcode() and equals() both) is same.

By the way, why I can not edit my comment here?

saytida said...

your articles are awesome..easy to understand.
Is there an article where you explain how hashset is implemented using a hashmap?
Also what is the internal working of hashmap, like it is a combination of array and linked list or what actually is a bucket, Map.Entry and all??
Can you please provide me the links to such articles?

Anonymous said...

1. When Hashset is just a wrapper which uses HashMap, How is it slower than HashMap?
2. When both classes do not have the same methods, how was this performance comparison done (Again add method of HashSet only delegates the call to a internal HashMap's put method)?

Anonymous said...

how hashmap works internally in java? could u please explain in detail

Kunal said...

Nice explanation

Unknown said...

Please add also examples.

Javin Paul said...

@Unknown, sure, will add some examples sooner.

Sanjeev Kulkarni said...

From the implementation of HashSet class, we can say that it internally stores the objects into HashMap into keys with a dummy object.

public class HashSet
extends AbstractSet
implements Set, Cloneable,

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

public HashSet() {
map = new HashMap<>();
// SOME CODE ,i.e Other methods in Hash Set

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

// SOME CODE ,i.e Other methods in Hash Set

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