What Is Internal Data Structure of HashSet?


Angela Bailey

What Is Internal Data Structure of HashSet?

The HashSet class in Java is a commonly used implementation of the Set interface. It provides an unordered collection of unique elements and offers constant-time performance for the basic operations, such as adding, removing, and searching for elements.

Internal Data Structure

The internal data structure used by a HashSet is an array of buckets. Each bucket can hold multiple elements and is essentially an instance of a linked list. The number of buckets in the array is determined by the initial capacity specified when creating the HashSet or when it needs to be resized due to a growing number of elements.

The actual element being stored in a HashSet is wrapped inside an instance of the Node class. This node contains a reference to the element itself and also a reference to the next node in the linked list (if any). In other words, each element is associated with a node that forms part of one of the linked lists within the buckets array.

Hashing Algorithm

To determine which bucket an element should be placed into, Java uses a hashing algorithm. When an element is added to the HashSet, its hash code is calculated using its own implementation of the hashCode() method. This hash code is then used as input to another hashing function that determines which bucket within the array should contain this element.

This hashing algorithm aims to distribute elements evenly across all buckets, minimizing collisions and ensuring efficient retrieval. However, collisions can still occur when multiple elements have the same hash code or are mapped to the same bucket. In such cases, Java uses a technique called chaining, where elements with colliding hash codes are stored in the same bucket as a linked list.

Performance Characteristics

The performance of a HashSet depends on the quality of the hashing algorithm and the distribution of elements. If a good hashing algorithm is used and elements are evenly distributed across buckets, the average time complexity for operations like adding, removing, and searching for an element is constant-time or O(1).

However, in the worst-case scenario where all elements collide and end up in the same bucket, the time complexity can degrade to linear or O(n), where n is the number of elements in the HashSet. This situation is highly unlikely but can occur if poorly implemented hash codes are used or if there is a high degree of hash code collisions.


In conclusion, a HashSet uses an array of buckets to store its elements. Each bucket contains a linked list of nodes, with each node representing an element. The internal data structure allows for efficient insertion, removal, and retrieval of elements due to its constant-time performance characteristics.

Understanding the internal data structure of HashSet helps developers utilize this class effectively and make informed decisions when choosing it over other Set implementations.

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