scala中伴随对象的问题

Question

以下代码编译良好（这是一个简单的伴随对象教程）

scala> :paste
// Entering paste mode (ctrl-D to finish)

trait Colours { def printColour: Unit }
object Colours {
private class Red extends Colours { override def printColour = { println ("colour is Red")} }
def apply : Colours = new Red
}

// Exiting paste mode, now interpreting.

defined trait Colours
defined object Colours

当我尝试时

val r = Colours

它工作正常，但是当我使用时

r.printColour 

我收到一个错误

<console>:17: error: value printColour is not a member of object Colours
   r.printColour

Answer 1

当您执行val r = Colours 时，它不会在伴随对象上调用您的apply，因为您的apply 方法不接受参数，因为它没有()。

看例子，

class MyClass {
  def doSomething : String= "vnjdfkghk"
}

object MyClass {
  def apply: MyClass = new MyClass()
}

MyClass.apply.doSomething shouldBe "vnjdfkghk" //explicit apply call

所以，你必须在你的伴生对象上调用 apply

val r = Colours.apply

否则，apply 必须有括号（empty-paren），这样就不需要显式调用.apply。

例如。

class MyClass {
  def doSomething : String= "vnjdfkghk"
}

object MyClass {
  def apply(): MyClass = new MyClass()
}

MyClass().doSomething shouldBe "vnjdfkghk"

非常有用的资源

Difference between function with parentheses and without [duplicate]

Why does Scala need parameterless in addition to zero-parameter methods?

Why to use empty parentheses in Scala if we can just use no parentheses to define a function which does not need any arguments?

Answer 2

在您的 apply 方法定义中添加括号 ...

def apply(): ...

...和Colours 对象调用。

val r = Colours()

然后它将按需要工作。

r.printColour  // "colour is Red"