Kotlin-具有通用返回类型的抽象函数

Question

I am new to Kotlin and really worried whether I am writing proper syntax or not. I have a super class and I want to write a generic abstract function as below.

abstract class A {
    abstract fun <T> getText() : Test<T>
}

class B : A() {
    override fun <T> getText(): Test<T> {
        return Test1() // Error - Required Test<T>, found Test1
    }
}

class C : A() {
    override fun <T> getText(): Test<T> {
        return Test2() // Error - Required Test<T>, found Test2
    }
}

class Test1 : Test<String>() {

}

class Test2 : Test<Int>() {

}

Can I solve this kind of problem with some sort of proper syntax. I guess I am doing some mistake? Can anybody please help?

Answer 1

What you're doing cannot be correct, because it would allow the following code:

val test: Test<Int> = B().getText()

B().getText() tries to return Test1() , which is Test<String> . But because the return type can be anything, its valid. This breaks the type system.

Answer 2

This is the answer you're looking for:

abstract class A<T> {
    abstract fun getText() : Test<T>
}

class B : A<String>() {
    override fun getText(): Test<String> = Test1()
}

class C : A<Int>() {
    override fun getText(): Test<Int> = Test2()
}

class Test1 : Test<String>()
class Test2 : Test<Int>()
open class Test<T>

You just need to move parameter T from the method signature to the class.

Bonus: I updated the above snippet to more idiomatic Kotlin

Answer 3

You likely want to enforce the type from the class rather than the method, due to Kiskae's example.

For example:

abstract class A<T> {
    abstract fun getText() : Test<T>
}

can be extended using

class B : A<String>() ...

which will match the type for Test1