斯卡拉蛋糕模式

Question

我试图将我对蛋糕模式的理解转换为简单的scala代码,并发现它没有编译.有人可以看看下面的代码,并告诉我这是什么方式我理解模式的问题？我读了这篇文章并尝试了类似的东西(http://www.cakesolutions.net/teamblogs/2011/12/19/cake-pattern-in-depth)

在下面的代码中 - println("This is " + userServiceComponent.whatCalc1) //> This is ()我期待它打印This is ScifiCalc Calc但是它的打印This is ()

码:-

trait Calc {
  def whatCalc
}

trait NormalCalc extends Calc {
  def whatCalc = new String("Normal Calc")
}

trait ScifiCalc extends Calc {
  def whatCalc = new String("ScifiCalc Calc")
}

trait TestTrait{
  def whatCalc1
}

trait TestCalc extends TestTrait {  
  this: Calc =>;

  def whatCalc1 = {
    whatCalc
  }
}

object SelfReferenceExample {
  println("Welcome to the Scala worksheet") 
  val userServiceComponent = new TestCalc with ScifiCalc {}
  println("This is " + userServiceComponent.whatCalc1) //> This is ()
}

Answer 1

Scala不是动态语言,它是静态类型的.当您在特征中做出此声明时Calc:

def whatCalc

缺少一个类型导致Scala默认它的类型为Unit.当此方法被覆盖时,类型保持不变Unit,因此字符串值被丢弃.Unit有一个实例,这()就是正在打印的内容.

如果将该行更改为def whatCalc: String,则应该可以正常工作.