如何模拟Kotlin Singleton类？

Question

如何模拟Singleton类？ 单例课程如下：

class Singleton private constructor() {
    init { println("This ($this) is a singleton") }    

    private object Holder { val INSTANCE = Singleton() }

    companion object {
        val instance: Singleton by lazy { Holder.INSTANCE }
    }
    var b:String? = null
}