Scala 专题教程-参数化类型(5): 类型上界

本篇介绍类型上界，我们使用合并排序算法来给人名排序，这里先定义一个Person类，它派生于Ordered Trait，定义如下：

class Person(val firstName:String, val lastName:String) 
	extends Ordered[Person]{
	def compare(that:Person) ={
		val lastNameComparison=
			lastName.compareToIngnoreCase(that.lastName)
		if(lastNameComparison!=0)
			lastNameComparison
		else
			firstName.compareToIngnoreCase(that.firstName)
	}
	
	override def toString= firstName + " " + lastName
 
}

我们先测试一下这个类对象之间的比较关系，注意Ordered Trait定义了对象之间的 >=, scala> val robert=new Person("Robert","Jones") robert: Person = Robert Jones scala> val sally = new Person("Sally","Smith") sally: Person = Sally Smith scala> robert < sally res1: Boolean = true scala> james == james1 res2: Boolean = false

我们定义merge sort 算法如下：

def orderedMergeSort[T <: Ordered[T]] (xs: List[T]):List[T] ={
    def merge(xs:List[T],ys:List[T]):List[T] =
      (xs ,ys ) match {
        case (Nil, _) => ys
        case (_,Nil) => xs
        case (x:: xs1,y :: ys1 ) =>
          if (x < y) x:: merge(xs1,ys)
          else y :: merge( xs,ys1)
      }
 
    val n = xs.length /2
    if(n==0) xs
    else {
      val (ys, zs)= xs splitAt n
      merge(orderedMergeSort(ys),orderedMergeSort(zs))
 
    }
  }

这个函数要求输入的参数的类型需要派生于Ordered trait,此时你需要使用类型上界，类型上界使用 <: ordered class="brush: scala; title: ; notranslate" title=""> scala> val people = List ( | new Person("Larry","Wall"), | new Person("Anders","Hejlsberg"), | new Person("Guido","van Rossum"), | new Person("Alan","Kay"), | new Person("Yukihiro","Matsumoto") | | ) people: List[Person] = List(Larry Wall, Anders Hejlsberg, Guido van Rossum, Alan Kay, Yukihiro Matsumoto) scala> val sortedPeople=orderedMergeSort(people) sortedPeople: List[Person] = List(Anders Hejlsberg, Alan Kay, Yukihiro Matsumoto, Guido van Rossum, Larry Wall)