In a HashMap collection it is very important for a key object to implement hashCode() method and equals() method. If hashCode() method returns same hashcode for all key objects then the hash collision will be high in HashMap. Also with same hashcode, we will get same equals method that will make our HashMap inefficient.

The problem arises when HashMap treats both outputs same instead of different. It will overwrite the most recent key-value pair with the previous key-value pair.

So it is important to implement hashCode() and equals() methods correctly for an efficient HashMap collection.