Top Interview Questions: Internal Working of HashMap and What New Changes Were Made in HashMap’s Internal Working in Java 8 — A Step-by-Step Breakdown

Java’s HashMap is a popular data structure for storing key-value pairs. Understanding its internals helps write efficient code and ace technical interviews. This article briefly covers how the put() method works, the Java 8 improvements in collision handling and resizing, and includes key interview questions to aid preparation.

The Internal Process of Inserting an Element into HashMap

When you call the put() method to insert a key-value pair, several key steps take place internally. Here's a detailed breakdown:

1. Key Hashing

• Step 1: The put() method computes the hash code of the key using the hash() method.
• Step 2: The hash() function applies bit manipulation to minimize collisions and ensure that hash codes are evenly distributed.

int hash = hash(key);

2. Index Calculation

• Step 3: The HashMap uses the computed hash code to determine the bucket index where the key-value pair should be stored. The index is calculated using:

index = (n - 1) & hash;

Here, n is the size of the underlying array, and the bitwise AND operation ensures that the index is within the array bounds.