CODEWITHSUNDEEP
genericskotlinstatic
Hawklike
Hawklike

Reputation: 1211

How to make a Kotlin generic method static?

I know that it is possible to write a static generic method in Java.

I tried to implement my own solution in Kotlin but I failed.

class Wrapper<T>
{
    companion object <T> // error: Type parameters are not allowed for objects.
    {
        private var value: T? = null

        // implement some methods
    }
}

Is there any way how to implement a static generic method in Kotlin?

Upvotes: 2

Views: 3813

Answers (4)

Yogesh Umesh Vaity
Yogesh Umesh Vaity

Reputation: 48399

Companion object is a singleton

Generic type parameters of the class are not allowed inside the companion objects. The reason for this is that a single companion object is shared across all the instances of that class. This is true also for the different parameterized types of the same class. So, for example, the companion object is the same for whether it is a Wrapper<String> or a Wrapper<Int>. In other words, the T of the enclosing class cannot take different forms String or Int inside the companion object.

The type parameters can only be used with the member functions and member properties of the class. The value property in your example is not the member of the Wrapper class, it is a member of the companion object.

This is the same thing as saying:

class Wrapper<T> {
    class SomeOtherClass<T> {
        private var value: T? = null
        // implement some methods
    }
}

The difference is just a single object vs multiple objects of SomeOtherClass. The Ts are independent for Wrapper and SomeOtherClass.

That's it! Hope that points you in the right direction for the actual problem you are trying to solve.

Upvotes: 1

Stanislaw Baranski
Stanislaw Baranski

Reputation: 1418

companion object is not method. It's, in fact, an object that is companion to the Wrapper class, therefore it cannot be aware of the generic type of parent class. Same thing applies to Java:

public class Wrapper<T> {
    public static T value; //ERROR
}

Within this companion object you can declare methods and fields that will be static in java world.

class Wrapper<T>
{
    companion object
    {
        fun <T> genericMethod(){

        }
    }
}

But keep in mind that type of Wrapper<T> is not connected with type T in fun <T> genericMethod().

Upvotes: 7

Miha_x64
Miha_x64

Reputation: 6373

For Kotlin, there's no difference between static (top-level, companion) and instance functions/methods. So, the same generic function/method

fun <T> something() {
    // TODO
}

is valid both within a class body, on top-level, and in companion or object declaration.

Upvotes: 2

gpunto
gpunto

Reputation: 2852

You just need to specify the generic type parameter of the function. It doesn't matter where the function is, be it a class, a file, a companion object, etc.
Example with companion object:

companion object {
    fun <T> generic(t: T) {
        // do something generic
    }
}

Upvotes: 0

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