What data structure is used inside HashMap to store elements in Java?

Inside a HashMap in Java, elements are stored using an array, where each element of the array is a linked list (or a tree if the number of collisions in one bucket exceeds a threshold). Keys are hashed, and the hash code is used to determine the index in the array. If multiple keys map to the same index (collision), they are stored in a linked list or a balanced tree (starting from Java 8) to improve performance.