Difference between HashMap and HashSet in Java

HashMap vs HashSet is the most frequently asked question during any core java interview and the 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 the collection framework which allows us to work with a collection of objects. Collection Framework has its 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 the FCFS algorithm.

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

What is HashSet in Java ?

HashSetis the 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 is 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 to 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 the order of the objects but HashMap will not.HashMap allows null values and null keys.HashMap is not synchronized, but the 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 theDifference between HashMap and hashtable in Java

Difference between HashSet and HashMap in Java

Following are some differences between HashMap and HashSet:

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

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