Internal Working of HashMap in Java

balakrishnamakineni

5 min read

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Jan 15, 2026

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HashMap is one of the most commonly used and most frequently asked topics in Java interviews. It tests your understanding of:

✔ Data Structures (Hashing + Trees)
✔ Memory Layout
✔ Object equality
✔ Complexity & resizing

In this article, we will go from zero → source code level and make you interview confident.

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🚀 Why HashMap Exists

Before HashMap existed, if we stored key-value data in an array or list, searching would require:

O(n) time — Linear Search

But with HashMap, average lookup time becomes:

O(1) — Constant Time

This is achieved using hashing.

Very High-Level Idea

A HashMap converts a key → into a hash → then maps it into a bucket index → stores the key-value pair.

In code:

hash = hash(key)
index = hash % capacity

Understanding the Map Interface

Map is an interface:
The map defines essential operations: