Nullable Types — String? and the ? Operator

fun main() {
    val name: String  = "Alice"        // never null
    val maybe: String? = null          // explicitly nullable

    // name.length        // OK — compiler knows it can't be null
    // maybe.length       // ✗ compile error: receiver may be null

    // Three ways to handle the nullable safely:

    // 1. Safe call ?. — returns null if receiver is null
    val len: Int? = maybe?.length
    println(len)                         // null

    // 2. Elvis ?: — supply a fallback
    val len2: Int = maybe?.length ?: 0
    println(len2)                        // 0

    // 3. Not-null assertion !! — throws NPE if null (use sparingly)
    val len3: Int = maybe!!.length       // NullPointerException here

    // 4. Smart cast after a null check
    if (maybe != null) {
        println(maybe.length)            // smart-cast to non-null String
    }
}