Nrthugu

So I have an abstract class Composition, which has two children: one is a Track, and one is an Album (which is a group of Tracks).

class Composition(val name: String, ...)

class Track(name: String): Composition(name)

class Album(name: String, val tracks: List<Track>): Composition(name)

So far, so good. Now, I have the duration that is added. It is abstract in Composition, so I can override it in the children:

abstract class Composition(...){

    abstract fun getDuration(): Int

}

Now, I can add override the method in the Track, which takes it as a parameter:

class Track(..., private val duration: Int): Composition(...){

    override fun getDuration() = duration

}

And finally, I make the Album, whose duration is the sum of the Tracks:

class Album(..., val tracks: List<Track>): Composition(...){

    override fun getDuration() = tracks.sumBy { it.getDuration() }

}

It works as intended, but I do not understand why I cannot simply use tracks.sumBy { it.duration }, since in Kotlin properties are nothing more than getters and setters (I'm thinking about the getDuration in Composition).

I feel like I'm missing something, because if the same code was written in Java, I would be able to call composition.duration as a property -- so that makes me think that Kotlin allows it from Java code, but not from Kotlin code, which is sad.

An other example:

Let's say I have a class named Artist, who wrote multiple Compositions:

class Artist(

    val nom: String,

    private val _compositions: MutableList<Composition> = ArrayList()

) {



    // HERE (I wrote the extension method List<E>.toImmutableList)

    fun getCompositions() : List<Composition> = _compositions.toImmutableList()

}

This is standard in Java (exposing immutable versions of Collections via getters, so they are not modified) ; Kotlin doesn't recognize it though:

val artist = Artist("Mozart")

artist.getCompositions() // Legal

artist.compositions      // Illegal

I thought about making this a property, but:
- If I choose the type List<E>, I can override the getter to return the immutable list, but I cannot use regular methods (add...) as the List is immutable
- If I choose the type MutableList<E>, I cannot override the getter to return ImmutableList (which is a subclass of List that I wrote, and is obviously not a subclass of MutableList).

There's a chance I'm doing something ridiculous while there is an easy solution, but right now I cannot find it.

In the end, my question is: Why aren't manually-written getters considered properties when written from Kotlin?

And, if I'm mistaking, What is the expected way of solving both of these patterns?

If you want to use it as property, you should use Kotlin-way to override getter.

For example:

abstract class Composition(...){

    abstract val duration: Int

}



// You can use "override" in constructor

// val - is immutable property that has only getter so you can just 

// remove private modifier to make possible get it.

class Track(..., override val duration: Int): Composition(...){

    ...

}



class Album(..., val tracks: List<Track>): Composition(...) {

    override val duration: Int 

        get() = tracks.sumBy { it.duration }

}

Also there are may be case when you need mutable property that can be changed only inside of object. For this case you can declare mutable property with private setter:

class SomeClass(value: Int) {

    var value: Int = value

        private set

}

Read more in docs: https://kotlinlang.org/docs/reference/properties.html#getters-and-setters

You have to define duration as an abstract property and not as an abtract function (https://kotlinlang.org/docs/reference/properties.html#getters-and-setters):

abstract class Composition(val name: String) {

    abstract val duration: Int

}

class Track(name: String, override val duration: Int): Composition(name)

class Album(name: String, val tracks: List<Track>): Composition(name) {

    override val duration: Int

        get() = tracks.sumBy { it.duration }

}

The getter/setter conversion as properties does only work for Java classes (https://kotlinlang.org/docs/reference/java-interop.html#getters-and-setters).