I am confused by Kotlin generics. My understanding...

# getting-started

[voohbar]

I am confused by Kotlin generics. My understanding was that if I declare T generic argument it implicitely uses Any? as the upper boundary. Otherwise what would T : Any do? But then it is not letting me to set T to null.

interface Form<T> {
    
}

abstract class SpecificForm<T> {
    
    private var bean: T = null // does not compile
}

[voohbar]

why do I have to declare the bean as T? when T already is includes nullable types?

[Jeff Lockhart]

Because T could be a non-null type. It's not guaranteed to be nullable.

[voohbar]

here is a closer example:

interface Form<T> {

    fun getBean(): T
}

class GenericForm<T : Any?> : Form<T> {

    override fun getBean(): T {
        return null
    }
}

why can’t i return null here? the T is clearly declared as nullable

[voohbar]

my problem is that if I declare getBean() to return T? then non of the interface implementations can return non-nullable T 😞

[Joffrey]

No it's not declared as nullable. : Any? doesnt add any bounds to T

[Joffrey]

So someone could pass a non-nullable T still

[voohbar]

i understand.. they would build GenericForm as having String and then getBean would return null which does not conform to T

[Jeff Lockhart]

my problem is that if I declare getBean() to return T? then non of the interface implementations can return non-nullable T

Subclasses can override a function with a nullable return type to narrow the scope to return a non-null.

[voohbar]

cool, that does help! can’t override a method parameter that way though

[voohbar]

fun setBean(bean: T?) cannot be overridden as fun setBean(bean: T)

[Jeff Lockhart]

Correct

[voohbar]

thanks all gratitude thank you