Kotlin - How i can access my new class extension function from another file

Question

I am very new to Kotlin.

I want to be able to add a function to my abstract class, so when I define that function I will be able to invoke that on every child from that class(they inherit the abstract class)

However,I want to define those extension functions in other file. I can't access those functions when i try to invoke them on a particular child implementation of the abstract class.

What are the rules, that I need to made to resolve my problem?

I want to by able achieve something like this:

abstract class Parent(val val1, val val2, val val3){}

class Child(var val1, var val2, var val3) : Parent(val1, val2, val3){}

class Child2(var val1, var val2, var val3) : Parent(val1, val2, val3){}

The extension method for parent and all childs:

 fun Parent.convertToChild2( ): Child2? {

return //some creation method of Child2
} 

And I want to be able to invoke this:

child: Child
child.convertToChild2

I defined all classes in separate file and also the extension function in other file.

I cannot access the function like this - is not visible.

Dramatic

Answer 1

The answer for my question, that satisfies me is just to extract the method to some "object" structure in other file and whenever we want to access that function we must import the path(package.object.method) to this.

But the problem is, that IDE is not propose me the path to my extension function - i must import it by myself.

I am using Android Studio 3 preview, hope this will be fixed.

UPDATE

It is better to define those function in just plain Kotlin file, so the functions will be not owned by any structure. Then it will be not a problem with importing those automatically by IDE from any place.

Dramatic

Thank you that works don't forget to make the method public. – Guillerminaguillermo

The best way is to add the Extension Method in a separate new file. Don't write it inside any Class or Object. – Aguedaaguero

Answer 2

I believe that there is a misunderstanding here about extension functions. Extension functions are regular static functions that take in an instance of the receiver class as a parameter implicitly when you define the function and operate on it.

These static functions (aka extension functions) have some limitations.

They are not added to the class hierarchy so subclasses can not inherit them (if you define extension function for parent class you can't expect that method to be present in child class)

They don't have access to the private properties of the class that they are extending.

Also, they are resolved statically, for example (taken from here)

open class Shape

class Rectangle: Shape()

fun Shape.getName() = "Shape"

fun Rectangle.getName() = "Rectangle"

fun printClassName(s: Shape) {
  println(s.getName())
}     

printClassName(Rectangle())

This example prints "Shape", because the extension function being called depends only on the declared type of the parameter s, which is the Shape class.

Tager

Answer 3

First of all: There a many errors in your source code, you should fix before asking the question here (missing type annotations in constructors, missing val/var etc.).

To your question: Normally, you can access extension functions even if defined in different kt files. Read the official docs covering extension scopes.

It's important to import your extension function

Garey

Thanks for your answer, i was just asking on the concept not really dig into details. See my answer for the question if you want. – Dramatic

Ah okay sorry then. I've added the hint about importing – Garey

But the problem is, that IDE is not propose me the path to my extension function - i must import it by myself – Dramatic

Actually it does, at least IntelliJ is capable of doing so – Garey

Ok, I am using Android Studio 3 and also it is a preview version – Dramatic

But Android Studio should be to – Garey

I don't know, is not working in Android studio 3.0 Beta 6 – Dramatic

Answer 4

There is a way to do that:

fun <T: Parent> T.toChild2() {
    TODO()
}

Child(...).toChild2()

Jarman

You need to declare the generic type parameter before the function name to make it available in the receiver type expression. As explained here: kotlinlang.org/docs/extensions.html#extension-functions . – Epiphenomenon