Is access to a HashMap element guaranteed to be constant time?

Access to a HashMap element in Java is usually provided in amortized constant time O(1). This is achieved through the use of a hash function that distributes keys across buckets.

However, in the worst case, such as when there are many collisions (when different keys end up in the same bucket), access time can degrade to O(n), where n is the number of elements in the bucket. Modern implementations of HashMap in Java (starting from Java 8) use balanced trees (TreeNode) instead of linked lists when there are many collisions, which improves the worst case to O(log n).

Thus, in most cases, access to a HashMap element occurs in constant time, but in rare cases, it can be slower.