Getting a Scala Map from a Java Properties

Question

I was trying to pull environment variables into a scala script using java Iterators and / or Enumerations and realised that Dr Frankenstein might claim parentage, so I hacked the following from the ugly tree instead:

import java.util.Map.Entry
import System._

val propSet = getProperties().entrySet().toArray()
val props   = (0 until propSet.size).foldLeft(Map[String, String]()){(m, i) =>
  val e = propSet(i).asInstanceOf[Entry[String, String]]
  m + (e.getKey() -> e.getValue())
}

For example to print the said same environment

props.keySet.toList.sortWith(_ < _).foreach{k =>
  println(k+(" " * (30 - k.length))+" = "+props(k))
}

Please, please don't set about polishing this t$#d, just show me the scala gem that I'm convinced exists for this situation (i.e java Properties --> scala.Map), thanks in advance ;@)

Answer 1

Scala 2.10.3

import scala.collection.JavaConverters._

//Create a variable to store the properties in
val props = new Properties

//Open a file stream to read the file
val fileStream = new FileInputStream(new File(fileName))
props.load(fileStream)
fileStream.close()

//Print the contents of the properties file as a map
println(props.asScala.toMap)

Answer 2

Scala 2.7:

val props = Map() ++ scala.collection.jcl.Conversions.convertMap(System.getProperties).elements

Though that needs some typecasting. Let me work on it a bit more.

val props = Map() ++ scala.collection.jcl.Conversions.convertMap(System.getProperties).elements.asInstanceOf[Iterator[(String, String)]]

Ok, that was easy. Let me work on 2.8 now...

import scala.collection.JavaConversions.asMap
val props = System.getProperties() : scala.collection.mutable.Map[AnyRef, AnyRef] // or
val props = System.getProperties().asInstanceOf[java.util.Map[String, String]] : scala.collection.mutable.Map[String, String] // way too many repetitions of types
val props = asMap(System.getProperties().asInstanceOf[java.util.Map[String, String]])

The verbosity, of course, can be decreased with a couple of imports. First of all, note that Map will be a mutable map on 2.8. On the bright side, if you convert back the map, you'll get the original object.

Now, I have no clue why Properties implements Map<Object, Object>, given that the javadocs clearly state that key and value are String, but there you go. Having to typecast this makes the implicit option much less attractive. This being the case, the alternative is the most concise of them.

EDIT

Scala 2.8 just acquired an implicit conversion from Properties to mutable.Map[String,String], which makes most of that code moot.

Answer 3

In Scala 2.9.1 this is solved by implicit conversions inside collection.JavaConversions._ . The other answers use deprecated functions. The details are documented here. This is a relevant snippet out of that page:

scala> import collection.JavaConversions._  
import collection.JavaConversions._

scala> import collection.mutable._
import collection.mutable._
scala> val jul: java.util.List[Int] = ArrayBuffer(1, 2, 3)
jul: java.util.List[Int] = [1, 2, 3]
scala> val buf: Seq[Int] = jul
buf: scala.collection.mutable.Seq[Int] = ArrayBuffer(1, 2, 3)
scala> val m: java.util.Map[String, Int] = HashMap("abc" -> 1, "hello" -> 2)
m: java.util.Map[String,Int] = {hello=2, abc=1} 

Getting from a mutable map to an immutable map is a matter of calling toMap on it.

Answer 4

In Scala 2.8.1 you can do it with asScalaMap(m : java.util.Map[A, B]) in a more concise way:

var props = asScalaMap(System.getProperties())

props.keySet.toList.sortWith(_ < _).foreach { k =>
  println(k + (" " * (30 - k.length)) + " = " + props(k))
}

Answer 5

In Scala 2.13.2:

import scala.jdk.javaapi.CollectionConverters._

val props = asScala(System.getProperties)

Answer 6

Looks like in the most recent version of Scala (2.10.2 as of the time of this answer), the preferred way to do this is using the explicit .asScala from scala.collection.JavaConverters:

import scala.collection.JavaConverters._

val props = System.getProperties().asScala

assert(props.isInstanceOf[Map[String, String]])