I recently came to know that in Java 8 hash maps uses binary tree instead of linked list and hash code is used as the branching factor.I understand that in case of high collision the lookup is reduced to O(log n) from O(n) by using binary trees.My question is what good does it really do as the amortized time complexity is still O(1) and maybe if you force to store all the entries in the same bucket by providing the same hash code for all keys we can see a significant time difference but no one in their right minds would do that.
Binary tree also uses more space than singly linked list as it stores both left and right nodes.Why increase the space complexity when there is absolutely no improvement in time complexity except for some spurious test cases.
This is mostly security-related change. While in normal situation it's rarely possible to have many collisions, if hash keys arrive from untrusted source (e.g. HTTP header names received from the client), then it's possible and not very hard to specially craft the input, so the resulting keys will have the same hashcode. Now if you perform many look-ups, you may experience denial-of-service. It appears that there's quite a lot of code in the wild which is vulnerable to this kind of attacks, thus it was decided to fix this on the Java side.
For more information refer to JEP-180.