CODEWITHSUNDEEP
kotlin
Vojtěch
Vojtěch

Reputation: 12416

Kotlin overriding supertype variables

Consider two classes:

abstract class ParentRepository {}

class ChildRepository : ParentRepository {}

abstract class ParentClass {
    protected abstract var repository: ParentRepository
}

class ChildClass : ParentClass {
    override var repository: ChildRepository
}

The last part won't work:

override var repository: ChildRepository

It will complain: 

Type of 'repository' doesn't match the type of the overridden var-property 'protected abstract var repository: ParentRepository

I understand the problem, but I don't see the reason why this shouldn't work – ChildRepository is an instance of ParentRepository and this is a common thing I am used to from Java.

Upvotes: 9

Views: 9880

Answers (3)

qianlv
qianlv

Reputation: 550

Use keyword open:

protected open var repository: ParentRepository

Upvotes: 0

tynn
tynn

Reputation: 39843

You have to consider that in Kotlin properties are a set of getters and setters. While a val only has a getter, a var also defines a setter. Thus you'd be able to set repository to a ParentRepository instance which might not be an instance of ChildRepository.

If you consider the variances for generics, you'll see the same behaviors. While you can use covariance <out T> for getters, you have to use contravariance <in T> for setters.

Upvotes: 8

dipdipdip
dipdipdip

Reputation: 2556

You have to declare repository as val. You can still override it as var:

protected abstract val repository: ParentRepository

override var repository: ChildRepository

Upvotes: 18

Related Questions