nullability_1.kt

fun main() {
    var firstName: String = null // Error: You can't assign null to a non-null type


    // The question mark tells the compiler that this variable can hold null:
    val lastName: String? = null


    // You can't assign a nullable variable to a non-null type 
    // even though they're both Strings:
    firstName = lastName // Error: Type mismatch


    // You can't just call a method on a nullable variable
    // without making sure that it is not currently holding null.
    // This code won't compile:
    val result1 = lastName.substring(0, 2)


    // The classic null-checking works fine and this code will compile:
    if (lastName != null) {
        val result2 = lastName.substring(0, 2)
    }
}