CODEWITHSUNDEEP
kotlingenericstype-parameter
kc_dev
kc_dev

Reputation: 778

Kotlin - Put restriction on type parameter

Suppose we have an abstract class

abstract class BaseFragment

Further, I would like to add a type parameter on the BaseFragment class, like so

abstract class BaseFragment<T>

Now, here is where I'm starting to run into problems - I want to put a restriction on T, namely, I want T to be a subtype of BaseFragment. However, when I write

abstract class BaseFragment<T: BaseFragment>

I get a compilation error:

One type argument expected for class BaseFragment

How do I resolve this? I've tried BaseFragment<out T: BaseFragment> and
BaseFragment<T: BaseFragment<*>> but that doesn't seem to work either...

Upvotes: 1

Views: 299

Answers (1)

Tenfour04
Tenfour04

Reputation: 93511

You have to specify the type bounds of BaseFragment. <*> doesn't work, because then nothing can ever satisfy the condition. You can use

abstract class BaseFragment<T: BaseFragment<T>>

Subclasses can "pass the buck":

open class Foo<T: BaseFragment<T>>: BaseFragment<T>()
//or
open class Bar<T: Bar<T>>: BaseFragment<T>()

Or satisfy the subtype requirement themselves:

class Baz: BaseFragment<Baz>()

Upvotes: 1

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