How is HashMap organized inside Java?

HashMap in Java is implemented based on an array of buckets, each of which contains a linked list or a tree (TreeNode) of elements with the same hash.

Main points:

• Keys are hashed using the hashCode() method, then the hash is further mixed for even distribution.
• The bucket index is calculated as (hash & (capacity - 1)), where capacity is the size of the array.
• In case of collisions, elements are added to a linked list; if the list becomes longer than a certain threshold (usually 8), it is converted into a balanced tree (red-black), which improves search performance from O(n) to O(log n).
• When a certain load factor (usually 0.75) is reached, the array is resized and elements are redistributed.

This is roughly how the internal structure of HashMap is arranged, ensuring efficient key-based access.