如何在 Scala 中定义排序？
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How to define an Ordering in Scala?
提问by Ivan
Having val hm: HashMap[org.joda.time.DateTime, MyType]I am trying to get the first and the last DateTimeof the set by means of hm.keys.minand hm.keys.maxrespectively 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)?
有val hm: HashMap[org.joda.time.DateTime, MyType]我想获得第一和最后DateTime借助一组hm.keys.min，并hm.keys.max分别但是编译器说No implicit Ordering defined for org.joda.time.DateTime。如何定义这种排序（隐式和显式选项都很有趣）？
回答by retronym
object Joda {
implicit def dateTimeOrdering: Ordering[DateTime] = Ordering.fromLessThan(_ isBefore _)
}
// elsewhere
import Joda._
dateTimes.sorted
回答by Majki
To facilitate working with Joda DateTime in Scala, nscala-time was created: https://github.com/nscala-time/nscala-time
为了方便在 Scala 中使用 Joda DateTime，创建了 nscala-time：https: //github.com/nscala-time/nscala-time
After including it in your project with
将其包含在您的项目中后
libraryDependencies += "com.github.nscala-time" %% "nscala-time" % "1.8.0"
you can just import OrderingImplicits. Either all at once:
你可以只导入OrderingImplicits. 要么一次性：
import com.github.nscala_time.time.OrderingImplicits._
or a particular one:
或特定的：
import com.github.nscala_time.time.OrderingImplicits.DateTimeOrdering
