How does a HashMap data structure behave if all keys have the same hash code and the equals method always returns false when comparing different elements?

If all keys in a HashMap have the same hash code, then they will all be placed in the same bucket. Since the equals method always returns false for different keys, HashMap cannot determine that the keys are equal and will treat them as unique.

As a result:

• All elements will be stored in one chain (or a tree if the number of elements in the bucket exceeds a threshold).
• The performance of get/put operations degrades from amortized O(1) to O(n), where n is the number of elements in that bucket.

For example:

Map<Key, Value> map = new HashMap<>();
// All keys have the same hashCode and equals always returns false
map.put(key1, value1);
map.put(key2, value2);
// During get, a linear search will be performed in the chain
Value v = map.get(key1); // slow

Thus, poor implementation of hashCode and equals leads to a loss of HashMap efficiency.