Kotlin Programming: The Big Nerd Ranch Guide, 2nd Edition

10 Maps

The third commonly used type of collection in Kotlin is Map. The Map type has a lot in common with the List and Set types: All three group elements, are read-only by default, use parameterized types to tell the compiler the type of their contents, and support iteration.

Where Map is different from List and Set is that its elements consist of key-value pairs, and instead of index-based access using an integer, a map provides key-based access using a type that you specify. Keys are unique and identify the values in the map; values, on the other hand, do not need to be unique. In this way, Map shares another feature with Set: The keys of a map are guaranteed to be unique, just like the elements of a set.