HashMap Performance when overriding hashcode method

Question

In a HashMap, if I put custom objects as a key.

What would happen if I override hashCode() method and implement it to pass value as '1'; would there be any performance hit ?

if I change hashCode() method to return random value using Math.random() function what would happen to performance ?

Answer 1

Adding Math.random() doesn't affect much performance hit but it is a bad idea to construct hashcode values through random() function. Instead you can use some of the good hashing function to minimize the collision and which are much faster also. For reference you can check out some of the links http://www.partow.net/programming/hashfunctions/

Answer 2

If you are referring to the asymptotic time complexity then:

since HashMap uses hashCode to calculate which bucket to use in the hashtable if you return 1 from hashCode you effectively make your HashMap's performance be like an (unsorted) LinkedList's performance.

Returning random values will simply blow your HashMap up since equal objects will no longer have equal hashCodes.

Excerpt from Wikipedia:

+----------------------+----------+------------+----------+--------------+
|                      |  Insert  |   Delete   |  Search  | Space Usage  |
+----------------------+----------+------------+----------+--------------+
| Unsorted linked list | O(1)*    | O(1)*      | O(n)     | O(n)         |
| Hash table           | O(1)     | O(1)       | O(1)     | O(n)         |
+----------------------+----------+------------+----------+--------------+

So to sum it up you lose:

• Time complexity when searching your HashMap (from O(1) to O(n))
• lookup in your HashMap (it won't work anymore)