The “Force” Operator (!!)

Kotlin also includes an operator generally known as the Force Operator. It lets you convert a nullable type to its non-nullable equivalent, such as converting a String? to a String, and it should only be used when you are 100% certain that a nullable type isn’t null.

The official name of this operator is the “Non-null assertion operator.”


If you’re 100% certain that a nullable type doesn’t contain a null value, you can convert it to its non-nullable equivalent, like this:

> val s1: String? = "Christina"

> val s2: String = s1!!

> s2

However, beware that if the nullable type does contain a null value you’ll get a null pointer exception during this process:

> val s1: String? = null

> val s2: String = s1!!

Check for null before using !!

Getting a null pointer exception defeats the entire purpose of using nullable types, so the typical way of using the !! operator is to test that the nullable variable isn’t null before doing the assignment, like this:

var nullableName: String? = null

// nullableName may be reassigned ...

var name = ""

if (nullableName != null) {
    name = nullableName!!

Key points

A few key points about the Force operator:

  • !! gives you a way to convert a nullable type to its non-nullable equivalent
  • Be careful, it will throw a kotlin.KotlinNullPointerException if the value is null

And one note about when you should use the Force operator:

  • Rarely!!

In general, there are better ways to use nullable types rather than converting them all to their non-nullable equivalent.

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