【转】Scala: Example use for early definition / early initializer / pre-initialized fields-优快云博客

本文介绍了Scala编程语言中trait和class定义中使用抽象值预初始化的概念，包括如何直接使用预初始化的抽象值以及类定义中预初始化字段的实例。

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原文链接 http://stackoverflow.com/questions/16348541/scala-example-use-for-early-definition-early-initializer-pre-initialized-fi#

Let's see a example from the Programming in Scala book (page 451). If we have a definition like this:

trait RationalTrait {
   val numerArg: Int
   val denomArg: Int
}

Then numerArg and denomArg are called abstract vals & the trait can be used directly without extends, like this:

val x = new RationalTrait {
   val numerArg = 1
   val denomArg = 2
}

val y = new {
   val numerArg = 1
   val denomArg = 1
} with RationalTrait

The above two are both valid Pre-initializing of abstract val in trait, except that when you need to put an expression value to abstract vals, you can only use the later form, like this:

val z = new {
  val numerArg = 1 * x
  val denomArg = 2 * x
} with RationalTrait

Another interesting example in book is to Pre-initialized fields in a class definition.

class RationalClass(n: Int, d: Int) extends {
  val numerArg = n
  val denomArg = d
} with RationalTrait {
  def + (that: RationalClass) = new RationalClass(
    numer * that.denom + that.numer * denom,
    denom * that.denom
  )
}