Why should I override hashCode() when I override equals() method?

Question

Ok, I have heard from many places and sources that whenever I override the equals() method, I need to override the hashCode() method as well. But consider the following piec

Answer 1

Because HashMap/Hashtable will lookup object by hashCode() first.

If they are not the same, hashmap will assert object are not the same and return not exists in the map.