Understanding HashMap Internals and Building a Miniature HashMap in Java

HashMap is a widely used collection in Java, and its underlying concepts can help solve many business problems; this article first analyzes the fundamental design of HashMap and then demonstrates how to implement a miniature version.

The design consists of three key elements: a hash function, an array, and a singly linked list for handling collisions. A good hash function should be fast (using bit operations) and produce a uniform distribution to minimize collisions. When collisions occur, the classic approach is chaining via linked lists, though linear probing is also mentioned as an alternative.

To deepen understanding, the article defines a MyMap interface exposing fast put and get operations, with an inner Entry interface representing map entries. The implementation includes an array for storage, an initial capacity, and a load‑factor threshold for resizing.

The MyHashMap constructor initializes the array and demonstrates a façade pattern by providing two public constructors that delegate to a common initialization method.

The Entry class embodies the linked‑list node, illustrating how HashMap stores entries in buckets.

The put method is explained step by step: checking whether resizing is needed, creating a new array and rehashing if necessary, computing the bucket index from the key's hash, and either inserting a new entry or updating an existing one by traversing the linked list.

The article also presents the hash function used in the miniature implementation and compares it with the JDK's hash function, emphasizing the importance of bit manipulation for uniform distribution.

Resizing and rehashing are covered, noting that frequent resize/rehash operations can degrade performance because they involve creating a larger array and moving each existing entry.

The get method is straightforward: compute the bucket index, then traverse the linked list using == or equals to locate the matching key.

A test program demonstrates storing and retrieving values with the custom MyHashMap, showing the expected output and confirming that the miniature implementation works correctly.

Overall, the article provides a clear, step‑by‑step guide to the internal mechanics of HashMap and how to recreate a simplified version in Java.