Nullable Types

fun main() {
    var name: String = "Kotlin"
    var nickname: String? = null
    println("Name: $name, Nickname: $nickname")
}

fun printLength(text: String?) {
    if (text != null) {
        println("Length: ${text.length}")
    } else {
        println("Text is null")
    }
}

fun main() {
    printLength("Kotlin")
    printLength(null)
}

fun shout(text: String?) {
    if (text != null) {
        println(text.toUpperCase())
    }
}

fun main() {
    shout("hello")
    shout(null)
}

fun describeAge(age: Int?): String {
    return if (age == null) "Age unknown" else "Age is $age"
}

fun main() {
    println(describeAge(30))
    println(describeAge(null))
}

1. Forgetting the ? after a type when a value can legitimately be absent, causing the compiler to disallow assigning null to it.
2. Treating a nullable type like a regular one and calling a method directly, which the compiler rejects until the null case is handled.
3. Overusing nullable types everywhere out of caution, when many values could be designed to never be null in the first place.

• A type followed by ?, such as String?, can hold either a value of that type or null.
• A non-nullable type, like plain String, can never hold null; the compiler enforces this at compile time.
• Calling a member directly on a nullable type without handling the null case is a compile-time error.
• Kotlin's null safety catches many null pointer exceptions before the program ever runs.