Variable null on start and cannot be null after initialized

Is there any way to annotate variable in Kotlin which is null on application start and after it is created it cannot be assigned to null again?

null

null

I can't set field as e.g. var someField: Boolean? because it can be nulled at some point or lateinit var someField: Boolean because it is expected that field is initialized before it will be used (or maybe this is proper way to resolve my situation?).

var someField: Boolean?

lateinit var someField: Boolean

isInitialized

4 Answers
4

I'm not sure I understand why lateinit doesn't work for you, other than that it doesn't work for primitive types - but the notNull delegate might be what you're looking for:

lateinit

notNull

class X {
var z: String by Delegates.notNull()
}

val x1 = X()
x1.z = "foo"
println(x1.z) // foo

val x2 = X()
println(x2.z) // IllegalStateException: Property z should be initialized before get.


It will give you an exception if you read it before setting it, and otherwise the property will have a non-null type.

isInitialized

isInitialized

lateinit

null

null

I'm not sure because i don't really see what you want to do but why you don't create the var when you initialize it? So the var will never be null and you will not have to initiate it with null

If you really need this var in order to check if it's null to change it why you don't use an int instead of a bool ?

Declare it with latenit: lateinit var someField: Boolean

and after you assign a non-null value, use it like this someField!!
With this annotation you are saying to the compiler that somefield is not null, but it's your responsibility to keep it non-null.

lateinit var someField: Boolean

someField!!

somefield

null

When using a more complex type than Boolean, you should consider implementing a Null-Value-Pattern around it. Kotlin offers a neat structure for such a usecase:

Boolean

sealed class Base

object NotInitialized : Base()

class Initialized : Base()


Like this you wouldn't run into any nullability issues. Also you can define a default directly with your implementation.

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