Error how to fix?

Question

Kotlin

I don't know how to fix

import java.io.Serializable

class Point3D : Serializable {

    @Volatile
    @set:Synchronized
    var x: Int = 0
    @Volatile
    @set:Synchronized
    var y: Int = 0
    @Volatile
    @set:Synchronized
    var z: Int = 0

    constructor(pX: Int, pY: Int, pZ: Int) {
        x = pX
        y = pY
        z = pZ
    }

    constructor(pX: Int, pY: Int) {
        x = pX
        y = pY
        z = 0
    }

    constructor(worldPosition: Point3D) {
        synchronized(worldPosition) {
            x = worldPosition.x
            y = worldPosition.y
            z = worldPosition.z
        }
    }

    @Synchronized
    fun setTo(point: Point3D) {
        synchronized(point) {
            x = point.x
            y = point.y
            z = point.z
        }
    }

    override fun toString(): String {
        return "($x, $y, $z)"
    }

    override fun hashCode(): Int {
        return x xor y xor z
    }

    @Synchronized
    override fun equals(other: Any?): Boolean {
        if (other is Point3D) {
            val point3D = other
            var ret = true;
            synchronized(point3D) {
                ret = point3D.x == x && point3D.y == y && point3D.z == z
            }
            return ret 
        }
        return true
    }

    @Synchronized
    fun equals(pX: Int, pY: Int, pZ: Int): Boolean {
        return x == pX && y == pY && z == pZ
    }

    @Synchronized
    fun distanceSquaredTo(point: Point3D): Long {
        var dx = 0L
        var dy = 0L
        synchronized(point) {
            dx = (x - point.x).toLong()
            dy = (y - point.y).toLong()
        }
        return dx * dx + dy * dy
    }

    @Synchronized
    fun setXYZ(pX: Int, pY: Int, pZ: Int) {
        x = pX
        y = pY
        z = pZ
    }

    companion object {
        private const val serialVersionUID = 4638345252031872576L
    }
}