When it comes to storing key-value pairs in programming, the HashMap data structure is a popular choice. It provides efficient lookup, insertion, and deletion of elements. But have you ever wondered which data structure lies behind the scenes in a HashMap?
The Underlying Data Structure
In most programming languages, including Java, the underlying data structure used in a HashMap is an array. However, using just an array would not be sufficient for efficient operations on key-value pairs. To overcome this limitation, HashMap utilizes another data structure called Linked List.
Arrays and Linked Lists
An array is a collection of elements stored in contiguous memory locations. It provides constant time access to any element based on its index. However, searching for an element in an array requires iterating over each element until a match is found.
A Linked List, on the other hand, consists of nodes where each node stores both the value and a reference to the next node in the list. This allows for efficient insertion and deletion at any position within the list since only adjacent references need to be updated.
The Bucket Concept
In order to combine the benefits of arrays and linked lists, HashMap divides its internal array into multiple sections or buckets. Each bucket contains a linked list of key-value pairs that hash to that particular bucket.
This division into buckets helps reduce collisions – situations where two or more keys hash to the same bucket – by distributing the key-value pairs evenly across different parts of the array.
A key component of HashMap is its hashing function. This function takes the key as input and produces an index or hash code, which determines the appropriate bucket for the key-value pair.
Good hashing functions aim to evenly distribute keys across buckets to minimize collisions. This ensures efficient access to values based on their keys.
Although HashMap tries to minimize collisions, they can still occur due to the limited number of buckets compared to the number of possible hash codes. When a collision happens, new key-value pairs are appended to the linked list within that bucket.
If multiple key-value pairs exist in a single bucket, HashMap uses the key’s equals() method to find the specific pair based on their keys.
The choice of data structure in HashMap – combining arrays and linked lists – offers a trade-off between time complexity and space efficiency.
The use of arrays allows for constant time access using an index, resulting in O(1) complexity for most operations. However, linked lists provide flexibility for handling collisions but introduce extra memory overhead due to storing references between nodes.
In summary, HashMap uses an array-based data structure divided into buckets.
The hashing function determines which bucket is chosen based on the key’s hash code. In case of collisions, new pairs are added to the linked list within that bucket. This combination of arrays and linked lists provides efficient lookup, insertion, and deletion of elements in HashMaps.