在scala中输出映射值的问题

Question

I have the following code snippet:

val map = new LinkedHashMap[String,String]
map.put("City","Dallas")
println(map.get("City"))

This outputs Some(Dallas) instead of just Dallas . Whats the problem with my code ?

Thank You

Answer 1

Use the apply method, it returns directly the String and throws a NoSuchElementException if the key is not found:

scala> import scala.collection.mutable.LinkedHashMap
import scala.collection.mutable.LinkedHashMap

scala> val map = new LinkedHashMap[String,String]
map: scala.collection.mutable.LinkedHashMap[String,String] = Map()

scala> map.put("City","Dallas")
res2: Option[String] = None

scala> map("City")
res3: String = Dallas

Answer 2

It's not really a problem.

While Java's Map version uses null to indicate that a key don't have an associated value, Scala's Map[A,B].get returns a Options[B] , which can be Some[B] or None , and None plays a similar role to java's null .

REPL session showing why this is useful:

scala> map.get("State")
res6: Option[String] = None

scala> map.get("State").getOrElse("Texas")
res7: String = Texas

Or the not recommended but simple get :

scala> map.get("City").get
res8: String = Dallas

scala> map.get("State").get
java.util.NoSuchElementException: None.get
        at scala.None$.get(Option.scala:262)

Check the Option documentation for more goodies.

Answer 3

There are two more ways you can handle Option results.

You can pattern match them:

scala> map.get("City") match {
 |   case Some(value) => println(value)
 |   case _ => println("found nothing")
 | }
Dallas

Or there is another neat approach that appears somewhere in Programming in Scala . Use foreach to process the result. If a result is of type Some , then it will be used. Otherwise (if it's None ), nothing happens:

scala> map.get("City").foreach(println)
Dallas

scala> map.get("Town").foreach(println)