不帶參數(shù)

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

    private object Holder { private val INSTANCE = Singleton() }

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

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

    companion object {
      private  val instance: = Singleton()
    }
    var b:String? = null
}

帶參數(shù)

class Singleton private constructor(str: String) {
    var string: String = str;

    init {
        println("str is $str")
        println("string is $string")
    }

    companion object {
       @Volatile
        var instance: Singleton? = null

        fun getInstance(c: String): Singleton {
            if (instance == null) {
                synchronized(Singleton::class) {
                    if (instance == null) {
                        instance = Singleton(c)
                    }
                }
            }
            return instance!!
        }
    }
}