What is stored inside a bucket in a hash table data structure?

In the data structure Hash Table (for example, in Java HashMap), a bucket is an element of the array where all records (key-value pairs) with hash codes that fall into this bucket are stored.

Typically, inside a bucket, there is:

• A list or linked list (previously) of elements to resolve collisions when multiple keys have the same bucket index.
• In modern implementations (for example, in Java 8+), when there are many collisions, the linked list can be transformed into a balanced tree (TreeNode) to speed up search.

That is, a bucket contains a set of elements with the same index, implemented as a linked list or a tree, where each element stores a key, a value, and a reference to the next element (if it's a list).

Example: in Java HashMap, a bucket is either null (if empty) or a reference to the first node of a list or a tree of nodes.