What are the consequences of key collisions in a HashMap data structure?

HashMap key collisions occur when different keys have the same hash code and fall into the same bucket. The consequences of collisions:

• Performance: with many collisions, access time to elements worsens from O(1) to O(n) in the worst case, as elements in a single bucket must be iterated over.
• In Java 8 and above, to reduce the impact of collisions, when a certain threshold of elements in a bucket is reached, the linked list is transformed into a balanced tree (TreeNode), which improves search time to O(log n).

Thus, collisions affect the efficiency of HashMap, but modern implementations aim to minimize their negative impact.