Kotlin generics for functions in parameter and return type-CodePudding

everyone! I have some class from Java dependency:

public class MyClass {
   public MyClass copy() {
      ....
   }
   public void init() {
      ...
   }
}

And I want to write a function in my Kotlin class:

fun <T : MyClass> prepareListCopy(objects: List<T>?): List<T>? =
        objects?.map { it.copy().apply { init() } }

I want my function to accept and return List of subtypes of MyClass, but IDE shows me this error: Change return type of enclosing function 'KOtlinClass.prepareListCopy' to 'List< MyClass >?'

How can I correctly define generic in this function?

CodePudding user response：

In the definition you provided, MyClass.copy() returns MyClass. So even when you have a subtype (say MySubClass) and you call MySubClass.copy() you will still get a MyClass, not a MySubClass.

This is why you cannot say your generic function returns a List<T>?, you have to say it returns List<MyClass>?:

fun <T : MyClass> prepareListCopy(objects: List<T>?): List<MyClass>? =
        objects?.map { it.copy().apply { init() } }

(which is exactly what the error message is telling you :D)

Now given what this function does, there is no real point in knowing the exact T anymore, you could just say you want the input list to contain subtypes of MyClass by using List<out MyCLass>, and the function doesn't need to be generic:

fun prepareListCopy(objects: List<out MyClass>?): List<MyClass>? =
        objects?.map { it.copy().apply { init() } }