- Expression 1 does not compile: that's normal, be...

# getting-started

[olem]

- Expression 1 does not compile: that's normal, because in Kotlin function MutableMap<K, V>.put(key: K, value: V): V?, value requires String and receives null.
- Expression 2 compiles and throws NPE. I suppose it's because the Java function put in ConcurrentMap is called, so we loose null safety.
- Expression 3 does not compile. Why? Is it because the compiler knows the contract of ConcurrentHashMap and knows that values cannot be null? And how does it know this contract?
- Expression 4 does compile and throws NPE. I can't explain that because it's still a ConcurrentHashMap, this time with a Kotlin extension function.
- Expression 5 compiles and throws NPE. So after all, the Kotlin compiler does not know that ConcurrentHashMap cannot be null. So why did expression 3 not compile?
In any case, I know that I cannot put a null value in ConcurrentHashMap, but I'd like to understand why the Kotlin compiler cannot warn me (and why it does warn me under certain conditions).