What happens inside a HashMap when adding an element if the hashCode method returns the same value for all objects?

If the hashCode method returns the same value for all objects, then all elements added to the HashMap will fall into the same bucket. Inside this bucket, elements are stored as a linked list (or a tree if there are many elements and Java 8+ is used). This leads to insertion, search, and deletion operations degrading from amortized O(1) time to O(n), where n is the number of elements in that bucket.

Thus, the efficiency of HashMap is significantly reduced, as the hash function ceases to effectively distribute elements across buckets, and all elements end up in a single chain.