饿汉式

Kotlin中类没有静态方法，kotlin可以把类声明为一个对象，该对象与其他语言的静态成员是类似

object Single {
    fun logMsg(){
        println("this is a msg")
    }
}

懒汉式

class Single1 private constructor(){
    companion object{
        private var instance:Single1? = null
        fun getInstance():Single1{
            if(instance == null){
                instance = Single1()
            }
            return instance!!
        }
    }
}

双重校验

// 双重校验
class Single2{
    companion object{
        val instance by lazy(mode = LazyThreadSafetyMode.SYNCHRONIZED) { Single2() }
    }
}

静态内部类

// 静态内部类
class Single3{
    companion object{
        fun getInstance():Single3{
            return Inner.instance
        }
    }
    private object Inner{
        val instance = Single3()
    }
}