Kotlin：如何将集合传递给函数的泛型类型

Question

I have this abstract function:

    abstract fun <T> updateValue(value: T)

Now i want a subclass to override it with a collection into the generic type:

override fun <List<Int>> updateValue(value: List<Int>)

and this does not work. Basic types work just fine (String, Int etc) but when I want to use a collection I do not know the right syntax. This is ok:

override fun <String> updateValue(value: String)

Answer 1

You cannot override a generic function with a fixed parameter, instead, you may have the type parameter on a class or interface to make it. Eg

abstract class BaseClass<T> {
  abstract fun updateValue(value: T) : Unit
}

///class to accept _only_ List<Int>
class ListClass<List<Int>> {
  override fun updateValue(value: List<Int>) { /*...*/ }
}

/// class to accept _only_ String
class StringClass<String> {
  override fun updateValue(value: String) { /*...*/ }
}

The rationale is as follows when you declare a generic function, eg fun <T> updateValue(value: T) is mean it is supposed to work for every possible T . It will not do a pattern matching to find the best suitable match.

You may try checking types in the implementation of the generic function to support a specific case, but in general, it could be hard for collection classes because of type erasure .

fun <T> updateValue(value: T) {
  when (value) {
    is String -> handleStringValue(value)
    is List<*> -> handleListValue(value)
    else -> handleDefault(value)
  }
}

You cannot tell List<Int> from List<String> here. An inline fun with reified generics may help .

Answer 2

It seems like you'd actually want to use a generic parameter on the class instead:

abstract class Parent<T> {
    abstract fun updateValue(value: T)
}

And then extend it to:

class Child : Parent<List<Int>> {
    override fun updateValue(value: List<Int>)
}

The only reason that the override with String works

override fun <String> updateValue(value: String)

is that String is not a type in this context but the name of the generic parameter you used to name T before. List<Int> however is not a valid name.