In this article, we will discuss difference between HashMap and HashSet classes in detail i.e.; HashMap v/s HashSet

1. HashMap v/s HashSet:

HashMap	HashSet
HashMap implements Map interface	HashSet implements Set interface
Used to store key-value pairs using put method   Example: hm.put(key, value);	Used to store only unique objects using add method   Example: hs.add(object);
HashMap doesn’t allow duplicate keys but values can be duplicated	HashSet doesn’t allow duplicate objects
HashMap allows maximum of one null key but any number of NULL values allowed	HashSet allows maximum of one null object to be added
HashMap internally uses an array of Entry<K,V> objects	HashSet internally uses HashMap to store unique objects
Performance-wise, HashMap is faster than HashSet	Performance-wise, HashSet is slower than HashMap

Q) When to use HashMap ?

• HashMap stores key-value pairs which uses hashing technique to store key-value pairs where methods are NOT synchronized
• So, search operation is faster with multiple threads access
• So, if business requirement is to store key-value pairs for faster search operation or more number of search operation on the basis of keys; without concerning concurrent access of map
• Then, HashMap is the very apt choice

Q) When to use HashSet ?

• HashSet stores unique elements using hashing technique
• So, search operation is faster
• So, if business requirement is to store unique elements for faster search operation or more number of search operation without concerning insertion order
• Then, HashSet is the very apt choice

Related Articles:

• Map interface
• Entry interface
• HashMap class
• LinkedHashMap class
• IdentityHashMap class
• WeakHashMap class
• SortedMap interface
• NavigableMap interface
• TreeMap class
• Hashtable class
• HashMap vs LinkedHashMap
• HashMap v/s LinkedHashMap v/s TreeMap
• HashMap v/s HashSet
• HashMap v/s Hashtable
• Properties class

References:           

• https://docs.oracle.com/javase/tutorial/collections/intro/
• https://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Collection.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Map.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Map.Entry.html
• https://docs.oracle.com/javase/tutorial/collections/interfaces/map.html
• https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Set.html
• https://docs.oracle.com/javase/7/docs/api/java/util/class-use/HashSet.html
• https://docs.oracle.com/javase/7/docs/api/java/util/HashSet.html

Happy Coding !!
Happy Learning !!