Kotlin synchronized doesn't work properly - kotlin

Question

I'm trying to create a simple program, which is model of Brownian motion using concurrency (impurities randomly move left and right in cells). I have Impurity and Cells classes. Cell class contains cell array which mean how many impurities in each cell at the moment. Each Impurity object changes cell array in Cells in own thread. I'm starting threads and they are running in infinite loop for 1 seconds. But before and after this I print sum of impurities in cells and these values not equal, which means I do something wrong with synchronisation. Here is code:
Cells class:
object Cells {
var cell = Array(N) { 0 }
fun addImpurity(impurity: Impurity) {
cell[impurity.currentCell]++
}
#Synchronized
fun move(impurity: Impurity, direction: Direction) {
if (direction == Direction.LEFT && impurity.currentCell > 0) {
cell[impurity.currentCell]--
cell[impurity.currentCell - 1]++
impurity.currentCell--
} else if (direction == Direction.RIGHT && impurity.currentCell < N - 1) {
cell[impurity.currentCell]--
cell[impurity.currentCell + 1]++
impurity.currentCell++
}
Unit
}
fun printCells() {
for (c in cell)
print("$c ")
}
}
enum class Direction {
LEFT, RIGHT
}
Impurity class:
class Impurity(var currentCell: Int) {
private lateinit var thread: Thread
init {
Cells.addImpurity(this)
}
fun startMoving() {
thread = Thread {
while (true) {
if (random() > P)
Cells.move(this, Direction.RIGHT)
else
Cells.move(this, Direction.LEFT)
}
}
thread.start()
}
fun stopMoving() = thread.interrupt()
}
and Main:
const val N = 10
const val K = 15
const val P = 0.5
fun main(args: Array<String>) {
val impurities = ArrayList<Impurity>()
for (i in 1..K)
impurities.add(Impurity(0))
println(Cells.cell.sum())
startMoving(impurities)
Thread.sleep(1000)
stopMoving(impurities)
Cells.printCells()
println(Cells.cell.sum())
}
private fun startMoving(impurities: ArrayList<Impurity>) {
for (impurity in impurities)
impurity.startMoving()
}
private fun stopMoving(impurities: ArrayList<Impurity>) {
for (impurity in impurities)
impurity.stopMoving()
}
Thanks in advance!

Answer 1

I think it might be better to manually signal to the thread that it should finish its work by having it contain some flag that it refers to in order to know when to quit the loop. For example:
class Impurity(var currentCell: Int) {
...
private var _continue = true
fun startMoving() {
thread = Thread {
while (_continue) {
}
}
...
fun stopMoving() {
_continue = false
}
}
Additionally, you might also want to wait till the actual thread itself dies as part of the call to stopMoving. This will ensure that all the threads have definitely received the signal and quit their loops, before you call Cells.printCells. For example you could add this method to the Impurity class:
fun waitForEnded() = thread.join()
And you could update stopMoving in the main class to call this method after signaling to each thread to stop:
private fun stopMoving(impurities: ArrayList<Impurity>) {
for (impurity in impurities)
impurity.stopMoving()
impurities.forEach(Impurity::waitForEnded)
}