Difference between HashMap and HashSet in java

One of the common interview question is "What is difference between HashMap and HashSet"..Before we actually see differences,let me give you brief introduction of both.

HashMap

HashMap implements Map interface which maps key to value.It is not synchronized and is not thread safe.Duplicate keys are not allowed and null keys as well as values are allowed.

HashMap<Interger,String> employeeHashmap=new HashMap<Integer,String>();
employeeHashmap.put(1,"Arpit");
employeeHashmap.put(2,"John");  

HashSet

Hashtable implements Set interface which does not allow duplicate value.It is not synchronized and is not thread safe.

Hashtable<String> employeeSet=new Hashtable<String>();
employeeSet.add("Arpit");
employeeHashmap.add("john");  

add method is used to add element to HashSet.If It return true then element is added successfully but if it return false then you are trying to insert duplicate value.

public boolean add(Object o)

One main thing about HashSet is objects which we are going to add in HashSet must implement Hashcode() and equals() method so that we can check for duplicate values.If we are adding custom objects to HashSet then we must override() Hashcode() and equals() method according to our need.If we do not override then object will take default implementation which may not desirable.

HashMap vs HashSet:

Parameter	HashMap	HashSet
Interface	This is core difference among them.HashMap implements Map interface	HashSet implement Set interface
Method for storing data	It stores data in a form of key->value pair.So it uses put(key,value) method for storing data	It uses add(value) method for storing data
Duplicates	HashMap allows duplicate value but not duplicate keys	HashSet does not allow duplicate values.
Performance	It is faster than hashset as values are stored with unique keys	It is slower than HashMap
HashCode Calculation	In hash map hashcode value is calculated using key object	In this,hashcode is calculated on the basis of value object.Hashcode can be same for two value object so we have to implement equals() method.If equals() method return false then two objects are different.