?, !! and ?:

Learn safe call, non-null assertion and Elvis operators.

Safe call, non-null assertion and Elvis

Three operators work with null. ?. is the safe call: if the receiver is null the result is null. !! is the non-null assertion: if the receiver is null it throws NullPointerException. ?: is the Elvis operator: if the left is null it returns the right side.

val name: String? = null
name?.length       // null
name!!.length      // Exception!
name ?: "Guest"    // Guest

These three operators manage null-risk in one line. Which one you choose depends on what you want to do when the value is null.

Usage examples of the three operators

name?.length safely fetches the length; if name is null the result is null, no exception. name!!.length is for those who say "I am sure it is not null"; if you are wrong, the app crashes.

val len = name?.length     // Int?
val forced = name!!.length  // Int (or Exception)
val display = name ?: "Guest"   // String

The Elvis operator is especially handy when you want to give a default: name ?: "Guest" returns "Guest" when name is null, otherwise name itself.

Exercise Handle name with safe call and Elvis

Define a val name: String? = null. Print the safe length with println(name?.length) and the default name with println(name ?: "Guest") on separate lines.

Show example solution

fun main() {
    val name: String? = null
    println(name?.length)
    println(name ?: "Guest")
}

Test yourself

Match operator to behavior

?.

Safe call (null if null)
!!

Non-null assertion (exception if null)
?:

Elvis (default if null)
String?

Nullable type
val len = name?.length — what is len if name is null?
- 0
- null
- -1
- Throws an exception
Show answer

null — The safe call (?.) returns null on a null receiver; no exception.

Fill in the blank: Elvis default

val display = name ____ "Guest"

Show answer

val display = name ?: "Guest"

Next lesson: let

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies