How can I access a val from a data class in kotlin?

Question

So I want to access isInBounds

Navigation.kt:

data class Coordinate(val x:Int , val y:Int){

    val isInBounds = x >= 0 && y >= 0

    operator fun plus(other:Coordinate) = Coordinate(x + other.x, y + other.y)

}

But when I try to access it I get an unresolved reference.

Game.kt

private fun move(directionInput: String) = try {
    val direction = Direction.valueOf(directionInput.uppercase())
    val newPosition = direction.updateCoordinate(player.currentPosition)


    

    if (!newPosition.isInBounds){

    }
    else{

    }
} catch (e: Exception){

}

enum class Direction(private val  coordinate: Coordinate){
    NORTH(Coordinate(0 , -1)),     
    EAST(Coordinate(1 , 0)),     
    SOUTH(Coordinate(0 , 1)),     
    WEST(Coordinate(-1 , 0));      
    fun updateCoordinate(playerCoordinate:Coordinate) {         
        coordinate + playerCoordinate     
    } 
}

What does direction.updateCoordinate() look like?

Ben P.
– Ben P.

2022-04-14 16:11:23 +00:00
Commented Apr 14, 2022 at 16:11 — Ben P.
– Ben P., Commented Apr 14, 2022 at 16:11

Sweeper · Accepted Answer · 2022-04-14 19:09:37Z

2

The mistake is in Direction. Note that you did not specify a return type for updateCoordinate, and you used a block body for it. This means that it implicitly returns Unit. So updateCoordinate calculates coordinate + playerCoordinate, discards the result, and returns Unit. This causes newPosition to be Unit, and that is obviously not going to be have an isInBounds property.

Clearly that is not what you want. You should instead declare updateCoordinate with an expression body:

fun updateCoordinate(playerCoordinate:Coordinate) =
    coordinate + playerCoordinate

or if you like block bodies:

fun updateCoordinate(playerCoordinate:Coordinate): Coordinate {
    return coordinate + playerCoordinate
}

This is just my opinion, but I think it is more readable to have updateCoordinate as a method on Coordinate, called movedTowards:

data class Coordinate(val x: Int, val y: Int) {
    operator fun plus(coord: Coordinate) = ...

    fun movedTowards(direction: Direction) =
        this + direction.coordinate // you'd have to make direction.coordinate public
}

edited Apr 14, 2022 at 19:09

answered Apr 14, 2022 at 16:33

Sweeper

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1 Comment

Tenfour04 Over a year ago

A suggestion: In the standard library, functions that mutate an object have the present-indicative verb form, and functions that return a modified copy of an object have the past-participle verb form. So a more conventional name in this case would be "movedTowards".

Tenfour04 · Accepted Answer · 2022-04-14 16:33:06Z

Your updateCoordinate function doesn't actually accomplish anything because it calculates something without doing anything with the result, like returning it. Therefore, it implicitly returns Unit, so newPosition is just a reference to Unit.

Change the function to return the new coordinate:

fun updateCoordinate(playerCoordinate: Coordinate): Coordinate {         
    return coordinate + playerCoordinate     
}

I recommend giving this function a better name. The word "update" implies that it is mutating the instance that is passed to it instead of calculating a new one.

Logically, since this function is something that adds something to a coordinate, it would make more sense for legibility and easy reasoning of your code to invert it (swap the receiver and parameter). So I would pull this function out of the enum class and make it a Coordinate extension function.

fun Coordinate.moved(direction: Direction): Coordinate {
    return this + direction.coordinate
}

but you'll have to make the Direction's coordinate property public or internal.

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