Difference between HashMap and HashSet

Last Updated : 18 Mar, 2026

HashMap and HashSet are part of the Java Collection Framework used for efficient data storage and retrieval. They differ mainly in how data is stored and how uniqueness is maintained.

HashMap stores data in key-value pairs
HashSet stores only unique elements
Both use hashing for fast operations

HashMap

HashMap is a data structure that stores elements in key-value pairs, where each key is unique and maps to a value. It is widely used when we need to associate one value with another.

Allows one null key and multiple null values
Uses put() method to insert elements

Output

{1=C++, 2=Python}

HashSet

HashSet is a collection that stores only unique elements and does not allow duplicates. It is useful when we only care about distinct values.

Allows only one null value
Uses add() method to insert elements

Output

[Java, Python]

HashMap vs HashSet

HashMap	HashSet
Stores key-value pairs	Stores only unique elements
Keys are unique, values can repeat	No duplicate elements allowed
Allows one null key and multiple null values	Allows only one null value
Uses put() method	Uses add() method
Implements Map interface	Implements Set interface
Used for mapping data	Used for storing unique values