Show GUI with Swing in Kotlin

Main.kt

import java.awt.Color
import java.awt.Font
import java.awt.GridLayout
import java.awt.event.ActionListener
import java.awt.event.ActionEvent
import javax.swing.JButton
import javax.swing.JFrame
import javax.swing.JMenu
import javax.swing.JMenuBar
import javax.swing.JMenuItem
import javax.swing.Timer
import javax.swing.plaf.basic.BasicBorders.ButtonBorder

class ButtonListener(private val button: JButton) : ActionListener {
    override fun actionPerformed(event: ActionEvent) {
        button.background = Color.RED
    } // fun actionPerformed()
} // ButtonListener

enum class Direction {
    NORTH,
    WEST,
    SOUTH,
    EAST
} // Direction

class Game() {
    var board = listOf(
        MutableList<JButton>(4) { JButton() },
        MutableList<JButton>(4) { JButton() },
        MutableList<JButton>(4) { JButton() },
        MutableList<JButton>(4) { JButton() },
    )

    private var blankTileRow = 3
    private var blankTileCol = 3
    private val rng = kotlin.random.Random(System.nanoTime())

    init {
        val buttonBackgroundColor = Color(216, 160, 104)
        val shadow = Color(128, 160, 144)
        val darkShadow = Color(96, 80, 72)
        val highlight = Color(224, 236, 216)
        val lightHighlight = Color(248, 248, 192)

        val buttonBorder = ButtonBorder(
            shadow, darkShadow,
            highlight, lightHighlight
        )
        val buttonFont = Font("Arial", Font.BOLD, 48)

        for (row in 0 until 4)
            for (column in 0 until 4) {
                val button = board[row][column]
                val n = (row * 4) + column
                val string = String.format("%02d", n)
                button.text = if (n != 15) string else ""
                button.background = buttonBackgroundColor
                button.border = buttonBorder
                button.font = buttonFont
            } // for
    } // init

    fun move() {
        val possibleMoves = mutableListOf<Direction>()
        if (blankTileCol < 3) possibleMoves.add(Direction.EAST)
        if (blankTileCol > 0) possibleMoves.add(Direction.WEST)
        if (blankTileRow < 3) possibleMoves.add(Direction.SOUTH)
        if (blankTileRow > 0) possibleMoves.add(Direction.NORTH)

        val index = rng.nextInt(possibleMoves.size )
        var neighborsRow = blankTileRow
        var neighborsCol = blankTileCol

        when (possibleMoves[index]) {
            Direction.EAST -> neighborsCol++
            Direction.WEST -> neighborsCol--
            Direction.SOUTH -> neighborsRow++
            Direction.NORTH -> neighborsRow--
        } // when

        val neighborsText = board[neighborsRow][neighborsCol].text
        val blankTileText = board[blankTileRow][blankTileCol].text

        board[neighborsRow][neighborsCol].text = blankTileText
        board[blankTileRow][blankTileCol].text = neighborsText

        board[neighborsRow][neighborsCol].repaint()
        board[blankTileRow][blankTileCol].repaint()

        blankTileRow = neighborsRow
        blankTileCol = neighborsCol
    } // move()
} // Game

class GameBoard() : JFrame(), ActionListener {
    private val game = Game()

    init {
        this.setSize(512, 512)
        this.defaultCloseOperation = EXIT_ON_CLOSE

        this.jMenuBar = JMenuBar()

        val gameMenu = JMenu("# moves in game")
        gameMenu.font = Font("Arial", Font.ITALIC, 24)

        this.jMenuBar.add(gameMenu)

        for (n in arrayOf(8, 16, 32, 64, 128)) {
            val label = String.format("%04d moves", n)
            val menuItem = JMenuItem(label)
            menuItem.font = Font("Arial", Font.ITALIC, 24)
            gameMenu.add(menuItem)
        } // for

        val container = this.contentPane
        container.background = Color(112, 192, 224)

        container.layout = GridLayout(4, 4)

        for (row in 0 until 4)
            for (column in 0 until 4) {
                val button = game.board[row][column]
                button.addActionListener(ButtonListener(button))
                container.add(button)
            } // for
        this.isVisible = true
    } // init

    override fun actionPerformed( event: ActionEvent ) {
        game.move()
    } // actionPerformed( ActionEvent )
} // GameBoard

fun main() {
    val gameBoard = GameBoard()
    val timer = Timer(500, gameBoard)
    timer.start()
} // main()