第14章：ScalaのOption型とnullを語る ref: http://qiita.com/f81@github/items/7bca48469d9aea65780d

file0.scala

scala> val map = Map(1 -> "Moses", 2 -> "Lucas", 3 -> "Henderson", 5 -> null)
map: scala.collection.immutable.Map[Int,String] = Map(1 -> Moses, 2 -> Lucas, 3 -> Henderson, 5 -> null)

scala> map get 1
res0: Option[String] = Some(Moses)

scala> map get 3
res1: Option[String] = Some(Henderson)

scala> map get 4
res2: Option[String] = None

file1.scala

scala> map get 5
res3: Option[String] = Some(null)

file2.scala

scala> val person = map get 2
person: Option[String] = Some(Lucas)

scala> person.get
res3: String = Lucas

file3.scala

scala> val nonePerson = map get 4
nonePerson: Option[String] = None

scala> nonePerson.get
java.util.NoSuchElementException: None.get
    at scala.None$.get(Option.scala:313)
    at scala.None$.get(Option.scala:311)
    at .<init>(<console>:10)
    at .<clinit>(<console>)
    at .<init>(<console>:7)
    at .<clinit>(<console>)
    at $print(<console>)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:734)
    at scala.tools.nsc.interpreter.IMain$Request.loadAndRun(IMain.scala:983)
    at scala.tools.nsc.interpreter.IMain.loadAndRunReq$1(IMain.scala:573)
    at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:604)
    at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:568)
    at scala.tools.nsc.interpreter.ILoop.reallyInterpret$1(ILoop.scala:756)
    at scala.tools.nsc.interpreter.ILoop.interpretStartingWith(ILoop.scala:801)
    at scala.tools.nsc.interpreter.ILoop.command(ILoop.scala:713)
    at scala.tools.nsc.interpreter.ILoop.processLine$1(ILoop.scala:577)
    at scala.tools.nsc.interpreter.ILoop.innerLoop$1(ILoop.scala:584)
    at scala.tools.nsc.interpreter.ILoop.loop(ILoop.scala:587)
    at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply$mcZ$sp(ILoop.scala:878)
    at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:833)
    at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:833)
    at scala.tools.nsc.util.ScalaClassLoader$.savingContextLoader(ScalaClassLoader.scala:135)
    at scala.tools.nsc.interpreter.ILoop.process(ILoop.scala:833)
    at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:83)
    at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:96)
    at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:105)
    at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

file5.txt

$ scalac Option1.scala
$ scala Option1
Lucas
Not exist.

file7.txt

$ scalac Option2.scala
Option2.scala:7: warning: match may not be exhaustive.
It would fail on the following input: None
        o match {
        ^
one warning found

Option1.scala

object Option1 {

  def main(args: Array[String]): Unit = {
    val map = Map(1 -> "Moses", 2 -> "Lucas", 3 -> "Henderson")

    def check(o: Option[String]) {
      o match {
        case Some(s) => println(s)
        case None => println("Not exist.")
      }
    }

    val some = map get (2)
    val none = map get (4)

    check(some)
    check(none)
  }

}

Option2.scala

object Option2 {

  def main(args: Array[String]): Unit = {
    val map = Map(1 -> "Moses", 2 -> "Lucas", 3 -> "Henderson")

      def check(o: Option[String]) {
        o match {
          case Some(s) => println(s)
        }
      }

    val some = map get (2)

    check(some)
  }

}