Null Safety

Null references have been called the billion-dollar mistake. Kotlin tackles this head-on by making null impossible in normal types. If a value can be null, you have to say so explicitly — and then the compiler forces you to handle it safely.

Nullable types

Add a ? after the type to mark it as nullable. Without it, null isn't allowed.

fun main() {
  var name: String = "Alice"
  var nickname: String? = null

  name = "Bob"
  nickname = "Bobby"

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

String can never hold null. String? can hold a string or null. The compiler tracks this and won't let you call methods on a nullable type without handling the null case.

Safe call operator ?.

Use ?. to call a method or access a property only if the value is not null. It returns null otherwise — no crash.

fun main() {
  val text: String? = "Hello"
  val nullText: String? = null

  println(text?.length)
  println(nullText?.length)

  val city: String? = null
  val len = city?.length ?: 0
  println(len)
}
    

The safe call returns null if the receiver is null. The Elvis operator ?: provides a default value when the expression on the left is null.

Elvis operator ?: and not-null assertion !!

The Elvis operator gives you a fallback. The not-null assertion !! says "I know this isn't null" — but use it carefully.

fun main() {
  val input: String? = "Kotlin"

  val length1 = input?.length ?: -1
  println(length1)

  val length2 = input!!.length
  println(length2)

  val danger: String? = null
  // danger!!.length  // Would throw NullPointerException!
}
    

Use ?: when you want a sensible default. Avoid !! unless you are 100% sure the value isn't null — it's basically saying "trust me, compiler, I know what I'm doing."