Nullability: When a Value Might Not Exist

Sometimes there's just no value

Every pet in PawWalk has a name. But its age? A rescue dog's age is often a guess the owner never entered. And not every walker uploads a profile photo. Kotlin forces you to be honest about missing data: a value that might not be there gets a different type.

Double is always a number. Double? — read it as "nullable Double" — is either a number or null, Kotlin's word for "nothing here." In the real models file, Pet.ageYears is a Double? and Walker.photoUrl is a String? for exactly these reasons.

The ? is part of the type. String and String? are different types, and Kotlin will not let you use a String? as if the value were definitely there. That one rule eliminates the #1 crash in most other languages: using a value that doesn't exist.