Scala辅助构造函数

发布时间：2020-12-16 09:10:12 所属栏目：安全来源：网络整理

导读：以下是带有助理员的 Scala课程.我的问题标有** class Constructors( a:Int,b:Int ) {def this() = { this(4,5) val s : String = "I want to dance after calling constuructor" //**** Constuctors does not take parameters error? What is this compile e

以下是带有助理员的 Scala课程.我的问题标有**

class Constructors( a:Int,b:Int ) {

def this() = 
{
  this(4,5)
  val s : String = "I want to dance after calling constuructor"
  //**** Constuctors does not take parameters error? What is this compile error?
  this(4,5)

}

def this(a:Int,b:Int,c:Int) =
{ 
  //called constructor's definition must precede calling constructor's definition
  this(5)
}

def this(d:Int) 
// **** no equal to works? def this(d:Int) = 
//that means you can have a constructor procedure and not a function
{
  this()

}

//A private constructor
private def this(a:String) = this(1)

//**** What does this mean?
private[this] def this(a:Boolean) = this("true")

//Constuctors does not return anything,not even Unit (read void)
def this(a:Double):Unit = this(10,20,30)

}

你可以在上面的**中回答我的问题吗？例如Constuctors没有参数错误？这是什么编译错误？

解决方法

答案1：

scala> class Boo(a: Int) {
     |   def this() = { this(3); println("lol"); this(3) }
     |   def apply(n: Int) = { println("apply with " + n) }
     | }
defined class Boo

scala> new Boo()
lol
apply with 3
res0: Boo = Boo@fdd15b

首先,这个(3)是一个代表主要的构造函数.第二个(3)调用此对象的apply方法,即扩展为this.apply(3).观察上面的例子.

答案2：

=在构造函数定义中是可选的,因为它们没有真正返回任何东西.它们与常规方法具有不同的语义.

答3：

private [this]被称为对象 – 私有访问修饰符.一个对象不能访问其他对象的私有[this]字段,即使它们都属于同一个类.因此,它比私人更严格.观察下面的错误：

scala> class Boo(private val a: Int,private[this] val b: Int) {
     |   def foo() {
     |     println((this.a,this.b))
     |   }
     | }
defined class Boo

scala> new Boo(2,3).foo()
(2,3)

scala> class Boo(private val a: Int,private[this] val b: Int) {
     |   def foo(that: Boo) {
     |     println((this.a,this.b))
     |     println((that.a,that.b))
     |   }
     | }
<console>:17: error: value b is not a member of Boo
           println((that.a,that.b))
                                 ^

答案4：

与Ans 2相同.

（编辑：李大同）

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