When working with Java, you may come across two similar data structures - HashMap and HashSet. While they may seem similar, there are some key differences between them. Let’s dive into the details of each and explore their differences.

HashMap

HashMap is a key-value pair data structure that allows us to store and retrieve elements based on their keys. It uses hashing to store the keys and provides constant time complexity for insertion, retrieval, and deletion operations. Here are some important points about HashMap:

• It does not allow duplicate keys, meaning each key must be unique.
• It allows null values and null keys (although only one null key is allowed).
• It provides methods such as put(key, value), get(key), and remove(key) to manipulate the elements.
• The order of elements in a HashMap is not guaranteed, as it depends on the hash code and bucket index.

HashSet

HashSet is a collection that allows us to store and retrieve elements in no particular order but ensures uniqueness of elements. It is implemented using a HashMap internally, where the elements are stored as keys, and the corresponding values are PRESENT, a static dummy object. Here are some important points about HashSet:

• It does not allow duplicate elements, meaning each element must be unique.
• It does not maintain any order of the elements.
• It allows null values but does not allow duplicate null elements.
• It provides methods such as add(element), contains(element), and remove(element).

Differences

Now, let’s discuss the differences between HashMap and HashSet:

1. Purpose:
   • HashMap is used to store key-value pairs, where each key must be unique.
   • HashSet is used to store unique elements, without any key-value mapping.
2. Underlying Implementation:
   • HashMap internally uses a hash table to store key-value pairs, where the keys are unique.
   • HashSet internally uses a HashMap to store the elements, where the elements are stored as keys, and the corresponding values are PRESENT.
3. Order:
   • HashMap does not guarantee the order of elements. They are stored and accessed based on the hash code and bucket index.
   • HashSet does not maintain any order of elements. They are just stored as unique entries without any specific ordering.
4. Null Values/Keys:
   • HashMap allows null values and a single null key.
   • HashSet allows null values but does not allow duplicate null elements.

In conclusion, HashMap is used for key-value mapping, while HashSet is used for storing unique elements. Understanding the differences between these two data structures is essential for choosing the appropriate one based on your requirements.

References:

• Java HashMap Documentation
• Java HashSet Documentation

#java #datastructures