Kotlin Map with non null values

Question

Let say that I have a Map for translating a letter of a playing card to an integer

 val rank = mapOf("J" to 11, "Q" to 12, "K" to 13, "A" to 14)

When working with the map it seems that I always have to make a null safety check even though the Map and Pair are immutable:

val difference = rank["Q"]!! - rank["K"]!!

I guess this comes from that generic types have Any? supertype. Why can't this be resolved at compile time when both Map and Pair are immutable?

Answer 1

There is another method for getting not null value from map:

fun <K, V> Map<K, V>.getValue(key: K): V

throws NoSuchElementException - when the map doesn't contain a value for the specified key and no implicit default value was provided for that map.

but operator for get == map[] returns nullable.

operator fun <K, V> Map<out K, V>.get(key: K): V?

Answer 2

It is not about the implementation of Map (being it Kotlin or Java based). You are using a Map and a map may not have a key hence [] operator returns nullable type.