Nullable Types

Nullable types are an approach in Kotlin to help you eliminate null values and null pointer exceptions (the dreaded NullPointerException). Here are the main concepts.

Key points

These are the key points of this lesson:

  • A variable of type String can never be null
  • A variable of the nullable type String? can be null
  • The operations that can be performed on nullable types are very limited

I use String in those statements, but the same things can be said about every other type, including Int, Float, Person, etc.

Standard types can’t be null

Variables that are instances of standard Kotlin types can’t contain null values:

val> val s: String = null
error: null can not be a value of a non-null type String
val s: String = null

> val i: Int = null
error: null can not be a value of a non-null type Int
val i : Int = null

Similarly, variables that are instances of custom types can’t be null either:

> class Person (var name: String)

> val p: Person = null
error: null can not be a value of a non-null type Line_3.Person
val p: Person = null

Nullable types

A nullable type is a variation of a type that permits null values. You declare a type to be nullable by adding a question mark after it:

var s: String? = null

This simple addition to your type allows your variable to contain null values. Notice that no exceptions are thrown in the REPL when you declare a nullable type:

> var s: String? = null
> var i: Int? = null
> var p: Person? = null

Also notice that I intentionally declare those variables as var fields. I do this because they’re declared with null values, but at some point later in their lifetime they’ll presumably contain more useful values.


A brief summary so far:

  • Default Kotlin types cannot contain null values.
  • A nullable type is a variation of an existing type, and can contain a null value. The ? operator says, “This instance of this type is allowed to contain a null value.”

As mentioned in the previous chapter, you should write your code to never use null values. Allowing null values in your code is considered a “bad smell.” (When I look at code that permits null values I think, “How quaint, this is like code from 1996.”)

Places where you’ll use nullable types

You’ll use and encounter nullable types in several areas:

  • Variable assignment
  • Function parameters
  • Function return values
  • Converting a nullable type to its non-nullable equivalent

Here’s what those situations look like. You’ve already seen variable assignment:

var s: String? = null

This is a function parameter:

fun foo(s: String?) ...

This is a nullable type used as a function return value:

fun foo(): String? = ...

You may also need to handle the process of converting a nullable type to its non-nullable equivalent:

val x: String? = null
val y: String = x    // error: this won’t compile

If you attempt to write the code as shown, you’ll see this error message on the second line:

//error: type mismatch: inferred type is String? but String was expected

I’ll show how to handle this situation in the following lessons.

Nullable types are limited

Because a nullable type can be null, in order for them to be safe to work with, the methods you can call on them are intentionally very restricted. As you can imagine, if you try to call a function like length() on a null string, it will throw a null pointer exception. Therefore, a String? instance doesn’t have a length property:

> var s: String? = "fred"

> s.length
error: only safe (?.) or non-null asserted (!!.) calls are allowed
on a nullable receiver of type String?

A String? instance also doesn’t have a toUpperCase() function:

> s.toUpperCase()
error: only safe (?.) or non-null asserted (!!.) calls are allowed
on a nullable receiver of type String?

That error message is Kotlin’s way of saying, “String? doesn’t have this functionality.”

This is an important point: a String? isn’t the same as a String. For example, with Kotlin 1.5.2, a String instance has well over 100 functions that can be called on it, while a String? has only 13. You can think of String? as being a limited subset of a String.

Note 1: Nullability concepts are enforced by the compiler at compile-time.

Note 2: I’ll discuss the ?. and !!. operators you see referenced in those error messages in the lessons that follow.

Coming soon: Operators to help you

While you can work with nullable types manually by checking for null values in if/then expressions:

val len = if (s != null) s.length else 0

that code is verbose, and it only gets worse when you have several nullable types. Therefore, this style of code is considered a bad practice, and Kotlin has operators to help you work with nullable types. Those operators will be demonstrated in the lessons that follow.

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