Null safety

The billion-dollar mistake

null means "no value here". In Java and many other languages, any object variable can secretly be null, and calling a method on it crashes the program with a NullPointerException (NPE). For years this was the most common crash in Android apps. Tony Hoare, who invented the null reference in 1965, later called it his "billion-dollar mistake".

Nullable and non-nullable types

Kotlin splits every type in two. String means "always a real string". String? (with a question mark) means "a string or null". The compiler tracks the difference and will not let you use a String? as if it were a String.

fun main() {
    val name: String = "Asha"
    // val bad: String = null      // error: Null can not be a value of a non-null type String
    val nickname: String? = null   // fine: the ? allows null

    println(name.length)           // 4
    // println(nickname.length)    // error: only safe (?.) or non-null asserted (!!.) calls are allowed
}

Safe call: ?.

The safe call operator ?. calls a method or reads a property only if the value is not null. If it is null, the whole expression simply becomes null instead of crashing.

val nickname: String? = null
println(nickname?.length)          // null
val city: String? = "Mysuru"
println(city?.uppercase())         // MYSURU

Elvis operator: ?:

The Elvis operator ?: gives a default value when the left side is null. (Turn it sideways and it looks like Elvis Presley's hair.)

val nickname: String? = null
val shown = nickname ?: "Guest"
println("Hello, $shown")           // Hello, Guest

val len = nickname?.length ?: 0    // 0

Smart casts after a check

fun greet(name: String?) {
    if (name != null) {
        println(name.length)      // name is smart-cast to String here
    } else {
        println("No name")
    }
}

let: run code only when not null

val phone: String? = "9876543210"
phone?.let {
    println("Sending OTP to $it")   // runs only if phone is not null
}

The !! operator

!! (the not-null assertion) tells the compiler "trust me, this is not null". If you are wrong, the app crashes with a NullPointerException, exactly what null safety was meant to prevent. Use it almost never; prefer ?., ?: or an if check.

Common mistakes

• Sprinkling !! to silence errors. Each one is a possible crash.
• Making every variable nullable "just in case". Use non-null types wherever a value must exist; nullability should mean something.
• Forgetting that ?. returns a nullable result: nickname?.length is Int?, not Int. Add ?: to get a plain Int.
• Trusting values from Java libraries or JSON to never be null.