How can I have a HashMap with unique keys in java?

Question

How can I have a HashMap with unique keys in Java? Or even does this make any sense to have unique keys in HashMap or the keys are unique by default? I am a newbie. thx

Answer 1

A generic hashmap is usually implemented as an associative array, so let's say your array has N elements, from 0 to N-1, when you want to add a new (key, value) pair, what it's done behind the scenes is (just conceptually):

1. index = hash(key) mod N
2. array[index] = value

So, by construction, a key is mapped to one and one only array entry.

Please note that it's actually a bit more complex than this: I am ignoring on purpose things like collision handling, rehashing, etc, you may have a good general idea here https://en.wikipedia.org/wiki/Hash_table