What happens inside a HashMap when two keys lead to the same hash bucket?

When two different keys in a HashMap have the same hash code and fall into the same hash bucket, a collision occurs. In Java HashMap, this is handled as follows:

1. The bucket stores a linked list (or a tree if there are many elements).
2. When adding a new element with the same hash code, the keys are compared using the equals() method.
3. If the key already exists, the value is updated.
4. If the key does not exist, the new element is added to the list in that bucket.

Starting from Java 8, if a bucket contains too many elements (usually more than 8), the linked list is transformed into a balanced tree (TreeNode), which improves search performance from O(n) to O(log n).

Thus, HashMap effectively handles collisions, ensuring correct storage and retrieval of elements.