How can I use a Java List with Scala's foreach?

Question

I have tried to convert a java list to a Scala list without success using asInstanceOf, as my returned list from an android call is a java list.

val apList = (wfm.getScanResults:java.util.List[ScanResult])

Wish to do this so that I can use the (new Scala) list in a for comprehension as it doesn't seem to like using a java list in this construct giving me an error.

value foreach is not a member of java.util.List[android.net.wifi.ScanResult]
    for (ap<-apList) { .... }
         ^

Is their a way to use a Java list in a for/foreach, without coercing it? And if I have to coerce, how? as asInstanceOf invokes a r/t error when used in this way.

Great question. I was wondering why my for comprehensions weren't working (individual entries couldn't be extracted with the proper type even though the list used generics). — Jon Onstott, Aug 21 '14 at 02:40

score 27 · Accepted Answer · edited Oct 26 '17 at 11:38

27

There is a set of conversions under scala.collection.JavaConversions and scala.collection.JavaConverters packages. First one acts implicitly on collection, the last one intends to be used explicitly (you take java collection and turn it to the scala analog with .asScala method and vice versa with .asJava).

import scala.collection.JavaConversions._

scala> val xs = new java.util.ArrayList[Int]()
// xs: java.util.ArrayList[Int] = []
xs.add(1)
xs.add(2)
// java.util.ArrayList[Int] = [1, 2]

scala> for(x <- xs) println(x)
1
2

And by the way, the reason asInstanceOf doesn't work is because scala and java collections have entirely different hierarchy and class cast cannot be made. Scala list has no other similarities with Java list except one common predecessor: Object class, so it is no different from trying to cast List to, say, java.util.Random.

edited Oct 26 '17 at 11:38

Andrii Abramov

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answered Jul 03 '13 at 23:29

om-nom-nom

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Perfect, thank you – RESTfulGeoffrey May 06 '18 at 18:36
1

This answer is out of date. I use `import scala.jdk.CollectionConverters._` and `for(x <- xs.asScala)` – Albert Hendriks Jan 02 '20 at 17:45
@AlbertHendriks : scala 2.12 doesnt support your import may be its 2.13 how to do it for scala 2.12 – Ram Ghadiyaram May 03 '23 at 06:27
@RamGhadiyaram I suppose in 2.12 you can use Om-nom-nom's answer (otherwise I don't know), but it should be recommended to upgrade your Scala version, which is very old. – Albert Hendriks May 04 '23 at 07:21

How can I use a Java List with Scala's foreach?

1 Answers1