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.

https://www.javatpoint.com/equals-and-hashcode-in-java#:~:text=The%20equals()%20and%20hashcode,implementation%20of%20these%20two%20methods.