Hash Collision

It is possible for a hash function to give the same result for two distinct elements.

The first letter of both "Smith" and "Sanders" is "S", so if the hash function takes the first letter of the last name they will have the same hash.

We call this situation a "hash collision." All it means is that these two objects will definitely go into the same bucket.

That's allowed to happen, but if everything goes into the same bucket then there isn't much point to using a HashMap over an ArrayList.