How does a HashMap data structure behave if all keys produce the same hash code?

If all keys in a HashMap have the same hash code, then they will all fall into the same bucket. In this case, the HashMap will store elements as a linked list (or a balanced tree if the number of elements in the bucket exceeds a certain threshold, starting from Java 8).

This leads to a degradation of the performance of get and put operations from amortized O(1) to O(n) in the worst case, where n is the number of elements in that bucket. Therefore, the efficiency of HashMap heavily depends on the quality of hash code distribution of the keys.