 <p>This should be doable in O(n) time... A hash map is usually implemented as a large bucket array, the bucket's size is (usually) directly proportional to the size of the hash map. In order to retrieve the key set, the bucket must be iterated through, and for each set item, the key must be retrieved (either through an intermediate collection or an iterator with direct access to the bucket)...</p> <p>**EDIT: As others have pointed out, the actual keyset() method will run in O(1) time, however, iterating over the keyset or transferring it to a dedicated collection will be an O(n) operation. Not quite sure which one you are looking for **</p>