Nullable types

Learn String? and nullable types.

Nullable types: String?

In Kotlin a type does not accept null by default. val name: String = null is a compile error. For a variable to be null, the type gets a ? suffix: String?.

val name: String? = null   // ok
val city: String = null   // compile error

The ? mark tells the compiler the type is nullable; null-risk is tracked at compile time. This prevents most runtime NullPointerException crashes.

Why use nullable variables?

Sometimes a value is genuinely absent: a field not yet fetched from the database, an input the user left blank, or a lookup with no target. null states this "no value" case explicitly.

val nickname: String? = null
val realName: String? = "Ava"

A nullable type tells the programmer to manage the null-risk consciously. The ?., !! and ?: operators you will see in the next lessons are the tools for this management.

Exercise Print a nullable name set to null

Define a val name: String? = null and print it with println(name).

Show example solution

fun main() {
    val name: String? = null
    println(name)
}

Test yourself

What happens with val name: String = null?
- It is a compile error
- name becomes null
- A runtime exception is thrown
- It returns Unit
Show answer

It is a compile error — String (no ? suffix) cannot hold null; it is a compile error.

Fill in the blank: nullable type

val name: ____ = null

Show answer

val name: String? = null

Match expression to meaning

String

cannot be null
String?

can be null
?

Nullable mark
null

No value

Next lesson: ?, !! and ?:

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