在项目中，我们常常会遇到排序（或比较）需求，比如：对一个Person类
case class Person(name: String, age: Int) {
override def toString = {
"name: " + name + ", age: " + age
}
}
按name值逆词典序、age值升序做排序；在Scala中应如何实现呢？
1. 两个特质
Scala提供两个特质（trait）Ordered与Ordering用于比较。其中，Ordered混入（mix）Java的Comparable接口，而Ordering则混入Comparator接口。众所周知，在Java中
- 实现Comparable接口的类，其对象具有了可比较性；
- 实现Comparator接口的类，则提供一个外部比较器，用于比较两个对象。
Ordered与Ordering的区别与之相类似：
- Ordered特质定义了相同类型间的比较方式，但这种内部比较方式是单一的；
- Ordered则是提供比较器模板，可以自定义多种比较方式。
以下源码分析基于Scala 2.10.5。
Ordered
Ordered特质更像是rich版的Comparable接口，除了compare方法外，更丰富了比较操作（<, >, <=, >=）：
trait Ordered[T] extends Comparable[T] {
def compare(that: A): Int
def < (that: A): Boolean = (this compare that) < 0
def > (that: A): Boolean = (this compare that) > 0
def <= (that: A): Boolean = (this compare that) <= 0
def >= (that: A): Boolean = (this compare that) >= 0
def compareTo(that: A): Int = compare(that)
}
此外，Ordered对象提供了从T到Ordered[T]的隐式转换（隐式参数为Ordering[T]）：
object Ordered {
/** Lens from `Ordering[T]` to `Ordered[T]` */
implicit def orderingToOrdered[T](x: T)(implicit ord: Ordering[T]): Ordered[T] =
new Ordered[T] { def compare(that: T): Int = ord.compare(x, that) }
}
Ordering
Ordering，内置函数Ordering.by与Ordering.on进行自定义排序：
import scala.util.Sorting
val pairs = Array(("a", 5, 2), ("c", 3, 1), ("b", 1, 3))
// sort by 2nd element
Sorting.quickSort(pairs)(Ordering.by[(String, Int, Int), Int](_._2))
// sort by the 3rd element, then 1st
Sorting.quickSort(pairs)(Ordering[(Int, String)].on(x => (x._3, x._1)))
2. 实战
比较
对于Person类，如何做让其对象具有可比较性呢？我们可使用Ordered对象的函数orderingToOrdered做隐式转换，但还需要组织一个Ordering[Person]的隐式参数：
implicit object PersonOrdering extends Ordering[Person] {
override def compare(p1: Person, p2: Person): Int = {
p1.name == p2.name match {
case false => -p1.name.compareTo(p2.name)
case _ => p1.age - p2.age
}
}
}
val p1 = new Person("rain", 13)
val p2 = new Person("rain", 14)
import Ordered._
p1 < p2 // True
Collection Sort
在实际项目中，我们常常需要对集合进行排序。回到开篇的问题——如何对Person类的集合做指定排序呢？下面用List集合作为demo，探讨在scala集合排序。首先，我们来看看List的sort函数：
// scala.collection.SeqLike
def sortWith(lt: (A, A) => Boolean): Repr = sorted(Ordering fromLessThan lt)
def sortBy[B](f: A => B)(implicit ord: Ordering[B]): Repr = sorted(ord on f)
def sorted[B >: A](implicit ord: Ordering[B]): Repr = {
...
}
若调用sorted函数做排序，则需要指定Ordering隐式参数：
val p1 = new Person("rain", 24)
val p2 = new Person("rain", 22)
val p3 = new Person("Lily", 15)
val list = List(p1, p2, p3)
implicit object PersonOrdering extends Ordering[Person] {
override def compare(p1: Person, p2: Person): Int = {
p1.name == p2.name match {
case false => -p1.name.compareTo(p2.name)
case _ => p1.age - p2.age
}
}
}
list.sorted
// res3: List[Person] = List(name: rain, age: 22, name: rain, age: 24, name: Lily, age: 15)
若使用sortWith，则需要定义返回值为Boolean的比较函数：
list.sortWith { (p1: Person, p2: Person) =>
p1.name == p2.name match {
case false => -p1.name.compareTo(p2.name) < 0
case _ => p1.age - p2.age < 0
}
}
// res4: List[Person] = List(name: rain, age: 22, name: rain, age: 24, name: Lily, age: 15)
若使用sortBy，也需要指定Ordering隐式参数：
implicit object PersonOrdering extends Ordering[Person] {
override def compare(p1: Person, p2: Person): Int = {
p1.name == p2.name match {
case false => -p1.name.compareTo(p2.name)
case _ => p1.age - p2.age
}
}
}
list.sortBy[Person](t => t)
RDD sort
RDD的sortBy函数，提供根据指定的key对RDD做全局的排序。sortBy定义如下：
def sortBy[K](
f: (T) => K,
ascending: Boolean = true,
numPartitions: Int = this.partitions.length)
(implicit ord: Ordering[K], ctag: ClassTag[K]): RDD[T]
仅需定义key的隐式转换即可：
scala> val rdd = sc.parallelize(Array(new Person("rain", 24),
new Person("rain", 22), new Person("Lily", 15)))
scala> implicit object PersonOrdering extends Ordering[Person] {
override def compare(p1: Person, p2: Person): Int = {
p1.name == p2.name match {
case false => -p1.name.compareTo(p2.name)
case _ => p1.age - p2.age
}
}
}
scala> rdd.sortBy[Person](t => t).collect()
// res1: Array[Person] = Array(name: rain, age: 22, name: rain, age: 24, name: Lily, age: 15)
3. 参考资料
[1] Alvin Alexander, How to sort a sequence (Seq, List, Array, Vector) in Scala.