visitors

Item.kt

sealed interface Item {
    val weight: Int
    val stringRepresentation: String
}

class Box(override val weight: Int) : Item {
    override val stringRepresentation: String = "Box"

}

class Container(items: List<Item>) : Item {
    override val weight: Int = items.sumOf { it.weight } + 2
    // or ==>   override val weight: Int
    //        get() = items.sumOf { it.weight } + 2

    override val stringRepresentation: String = "Container"
}

class WeightCalculator {

    fun calculate(items: List<Item>) = items.sumOf { it.weight }
}

class PrettyPrinter(private val logger: Logger) {

    init {
        logger.config(Level.VERBOSE)
    }

    fun print(items: List<Item>) = items.forEach { logger.log(it.stringRepresentation) }

}

class XMLPrinter(private val xmlWriter: XMLWriter) {

    fun print(items: List<Item>) {
        xmlWriter.writeComment("writing all items")
        items.forEach {
            xmlWriter.addNode(it.stringRepresentation) // representing xml in parent-child format is inhibited for brevity
        }
        xmlWriter.writeComment("finish writing items")
    }
}

class XMLWriter {
    fun addNode(stringRepresentation: String) {
        TODO("Not yet implemented")
    }

    fun writeComment(s: String) {
        TODO("Not yet implemented")
    }
}

class Logger {
    fun config(level: Level) {
        TODO("Not yet implemented")
    }

    fun log(stringRepresentation: String) {
        TODO("Not yet implemented")
    }

}

enum class Level {
    VERBOSE
}