When is it preferable to use a HashMap in Java over a TreeMap?

Using a HashMap is preferable when you require faster average time complexity for operations. A HashMap provides average time complexities of O(1) for key lookups, insertions, and deletions due to its use of hashing. This makes it highly efficient for these operations as long as the hash function is well-distributed.

In contrast, a TreeMap, which is based on a red-black tree structure, has time complexities of O(log n) for similar operations. Therefore, for scenarios where performance is a priority and the order of elements is not important, a HashMap is the optimal choice. The efficiency of a HashMap in handling large amounts of data or frequent operations is a significant advantage compared to alternatives like a TreeMap.