Private Setters

Part of the Object Oriented Programming section of Coddy's Kotlin journey. Lesson 9 of 57.

Often outside code should read a property but not change it. private set keeps the getter public and makes only the setter private:

class Counter {
    var count = 0
        private set

    fun increment() {
        count++
    }
}

Inside main:

val c = Counter()
c.increment()
println(c.count)
// c.count = 99   // error: the setter is private

Output:

This fits values that may only change through a rule, such as a score that only grows through valid awards:

class Player(val name: String) {
    var score = 0
        private set
    private val _history = mutableListOf<Int>()
    val history: List<Int> get() = _history

    fun award(points: Int) {
        if (points <= 0) return
        score += points
        _history.add(points)
    }
}

Inside main:

val p = Player("Ada")
p.award(10)
p.award(-5)
p.award(3)
println(p.score)
println(p.history)

Output:

13
[10, 3]

The history above shows a common pattern: a private MutableList inside the class, exposed as a read-only List. Callers can read the items but have no add or remove to change them:

private val _items = mutableListOf<String>()
val items: List<String> get() = _items      // read-only view
// outside: items.add("x")                  // error: List has no add

A val never changes after creation, not even inside the class. var with private set stays changeable inside the class only. Choose val for values that are fixed for the whole life of the object:

class Order(val id: Int) {            // never changes
    var status = "new"
        private set                       // changes, but only inside Order
    fun ship() { status = "shipped" }
}

Challenge

Easy

Complete the class Elevator(val topFloor: Int). floor and trips can be read from outside but only changed inside the class (private set). goTo(target) moves between floor 0 and topFloor: a valid move to a different floor updates floor, adds 1 to trips and returns true; an invalid or same-floor move returns false.

The supplied code reads the top floor and then target floors. It prints floor 3 after a move or stay 3 when the move was refused, and finally trips: with the count.

Your code goes in Elevator.kt. Main.kt holds the supplied input/output code and cannot be edited.

Try it yourself

fun main() {
    // Supplied input/output code: keep it as it is
    val input = generateSequence(::readLine).toList()
    val lift = Elevator(input[0].toInt())
    for (line in input.drop(1)) {
        val moved = lift.goTo(line.toInt())
        println("${if (moved) "floor" else "stay"} ${lift.floor}")
    }
    println("trips: ${lift.trips}")
}