A HashMap in Java is a data structure that provides fast access to data through key-value pairs. It works by using a hash table to store the pairs, where each key is hashed to generate an index in an array, called a bucket. Here’s a brief overview of its functioning:

1. Hashing: When a key-value pair is added to the HashMap, the key is passed through a hash function to compute a hash code. This hash code is then used to determine the index of the bucket where the pair should be stored.
2. Buckets: Each index in the array corresponds to a bucket, which can store multiple key-value pairs in case of hash collisions. Initially, the array is sparse, but as more pairs are added, it may be resized and rehashed.
3. Handling Collisions: HashMap handles collisions using chaining. When multiple keys hash to the same index, they are stored in a linked list (or a balanced tree if the list gets too long) within the bucket. This ensures that the structure can handle collisions efficiently.
4. Operations:
   • Insertion: The key is hashed, and the pair is placed in the appropriate bucket.
   • Retrieval: The key is hashed, and the corresponding bucket is searched for the key.
   • Deletion: The key is hashed, the bucket is located, and the key-value pair is removed.

This structure allows for average-time complexity of O(1) for insertion, deletion, and retrieval operations, making HashMap a highly efficient and widely used data structure in Java.