如何在Scala中定义订购? [英] How to define an Ordering in Scala?

问题描述

具有val hm: HashMap[org.joda.time.DateTime, MyType]我试图分别通过hm.keys.min和hm.keys.max来获取集合的第一个和最后一个DateTime，但是编译器会说No implicit Ordering defined for org.joda.time.DateTime.如何定义这种顺序(隐式和显式选项都很有趣)?

Having val hm: HashMap[org.joda.time.DateTime, MyType] I am trying to get the first and the last DateTime of the set by means of hm.keys.min and hm.keys.max respectively but the compiler says No implicit Ordering defined for org.joda.time.DateTime. How to define this ordering (both implicit and explicit options are interesting)?

推荐答案

object Joda {
    implicit def dateTimeOrdering: Ordering[DateTime] = Ordering.fromLessThan(_ isBefore _)
}

// elsewhere
import Joda._
dateTimes.sorted

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