Why "!!" is required when converting the readline string value into int

Question

I am new to kotlin, and I have been doing research on the syntax of the language. It is to my understanding that in kotlin you can cast data types using integrated functions like :

.toInt() 

converting 3.14 to an integer :

3.14.toInt()

since it is known that the readline() function returns a string i am not sure why this syntax is correct:

fun main() {
    println("please enter a int:")
    val num1 = readLine()!!.toInt()
    println("one more")
    val num2 = readLine()!!.toInt()

    println("sum : ${num1 + num2}")
}

and this syntax is incorrect

fun main() {
    println("please enter a int:")
    val num1 = readLine().toInt()
    println("one more")
    val num2 = readLine().toInt()

    println("sum : ${num1 + num2}")
}

returns the error:

 Error:(5, 26) Kotlin: Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type String

Just looking for a bit more of an explanation on casting and how the syntax differs when it comes to the readline() function and functions alike.

Answer 1

The method readLine() returns a String? - the question mark means it can either be null or a String. In Kotlin, you need to handle instances with nullable type with either ? or !! when you're invoking a method onto that instance.

The difference is that ? only proceeds when the instance is not null, and !! forces it to proceed. The latter may give you a NullPointerException.

For example:

val num1 = readLine()?.toInt()
// Here, num1 could either be a String or null

val num1 = readLine()!!.toInt()
// if it goes to this next line, num1 is not null. Otherwise throws NullPointerException