What happens inside a HashMap data structure if all its elements have the same key?

If all elements in a HashMap have the same key, then effectively only one element will remain in the structure — the last one added with that key. HashMap does not allow duplicate keys, so when a new value is added with an existing key, the old value is overwritten.

Internally, HashMap uses a hash function to distribute elements across buckets. If all keys are identical, all elements will fall into the same bucket, turning the structure into a linked list or a tree (depending on implementation and size). This significantly degrades the performance of search and insert operations, as they become O(n) instead of O(1).

Example:

HashMap<String, Integer> map = new HashMap<>();
map.put("key", 1);
map.put("key", 2);
System.out.println(map.get("key")); // Will output 2

Here, the value 1 will be overwritten by the value 2 for the key "key".