What is a collision in HashMap?

A collision in HashMap occurs when two different keys produce the same hash value (hash code) and, consequently, try to occupy the same cell in the array.

To resolve collisions in Java HashMap, chaining is used: each cell in the array stores a linked list (or a tree if there are many elements), where all elements with the same hash are added. When searching or inserting, the hash index is first calculated, then the chain is traversed to find the desired key.

This allows efficient handling of data even during collisions, but performance can decrease with a high number of collisions, so it is important to choose a good hash function and maintain the size of the table.