Kotlin Cookbook

Chapter 2. Basic Kotlin

This chapter contains recipes that work with the fundamentals of Kotlin. They show you how to use the language without relying on specific libraries.

2.1 Using Nullable Types in Kotlin

Problem

You want to ensure that a variable is never null.

Solution

Define the type of a variable without a question mark. Nullable types also combine with the safe call operator (?.) and the Elvis operator (?:)

Discussion

The most attractive feature of Kotlin may be that it eliminates almost all possible nulls. In Kotlin, if you define a variable without including a trailing question mark, the compiler will require that value to be non-null, as in Example 2-1.

Example 2-1. Declaring a non-nullable variable

var name: String

// ... later ...
name = "Dolly" 
// name = null 

Assignment to a non-null string

Assignment to null does not compile

Declaring the name variable to be of type String means that it cannot be assigned the value null or the code won’t compile.

If you do want a variable to allow null, add a question mark to the type declaration, as in Example 2-2.