Kotlin Null Safety and Nullable Types

A Guide to Null Safety and Nullable Types in Kotlin

One of the key design decisions in Kotlin was to eliminate the Null Pointer Exception, a common pitfall in many programming languages. In this post, we’ll delve into how Kotlin deals with nulls and the benefits of its robust null safety features.

“Simplicity is the soul of efficiency.” – Austin Freeman

Nullability in Kotlin

In Kotlin, you can’t assign null to a variable by default. This helps prevent null pointer exceptions.

var name: String = "Kotlin"
name = null // This line will cause a compilation error

In this example, name can’t be null, so trying to assign null to it will cause a compilation error.

Nullable Types

If you want to allow nulls, you can declare a variable as nullable by adding a ? to its type.

var name: String? = "Kotlin"
name = null // This is okay

In this example, name is a nullable String, so it can hold null values.

Null Safety Operators

Kotlin provides a set of operators for dealing with nullable types.

Safe Call Operator ?.

The safe call operator ?. can be used to safely access a method or property of a nullable object.

var name: String? = "Kotlin"
println(name?.length) // Prints "6"

name = null
println(name?.length) // Prints "null"

Here, name?.length returns name.length if name is not null, and null otherwise.

Elvis Operator ?:

The Elvis operator ?: can be used to provide a default value in case a nullable expression is null.

var name: String? = null
val length = name?.length ?: 0
println(length) // Prints "0"

Here, name?.length ?: 0 returns name.length if name is not null, and 0 otherwise.

Advanced Null Safety

For more advanced null safety, Kotlin provides the not-null assertion operator !! and the let function.

Not-Null Assertion Operator !!

The not-null assertion operator !! converts a nullable type to a non-nullable type, and throws a NullPointerException if the nullable type is null.

var name: String? = null
println(name!!.length) // Throws NullPointerException

This is a risky operator, and should only be used when you’re sure the variable isn’t null.

The let Function

The let function can be used with the safe call operator to perform an operation only when a variable is not null.

var name: String? = "Kotlin"
name?.let {
    println(it.length) // Prints "6"

Here, it inside the let block refers to the non-null value of name.


Kotlin’s approach to null safety and nullable types is a huge leap towards safer and more reliable code. It may require a bit of a mindset shift if you’re coming from a language where null is thrown around liberally, but it pays off in fewer null pointer exceptions and more predictable code.

As always, remember that mastering a programming language is a marathon, not a sprint. Take your time to understand each concept, and don’t rush. Keep practicing and exploring Kotlin, and soon, you’ll find dealing with nulls to be a breeze. Happy coding!