Unable to iterate Java List in Scala

Question

I'm using the Java Twitter4J library in a Scala project.

I'm calling the method

twitter.getFriendsStatuses()

This method returns a list of twitter4j.User objects containing statuses.

I try to iterate over them and it goes in an infinite loop over the first element:

val users:List[User] = twitter.getFriendsStatuses(userId, paging.getSinceId())
while( users.iterator.hasNext() ) {
  println(users.iterator.next().getStatus())
}

Any ideas?

score 27 · Accepted Answer · answered Mar 14 '11 at 13:38

27

I guess users.iterator produces the new iterator each time it's evaluated. Try this:

val it = users.iterator
while(it.hasNext() ) {
   println(it.next().getStatus())
}

answered Mar 14 '11 at 13:38

axtavt

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Thanks. Why didn't I think of that! :-) – Sri Mar 14 '11 at 13:49

score 18 · Answer 2 · answered Mar 14 '11 at 15:21

If you use Scala 2.8, you could use JavaConversion to convert Java collection to Scala collection automatically.

Ex.

import scala.collection.JavaConversions._

// Java Collection
val arrayList = new java.util.ArrayList[Int]
arrayList.add(2)
arrayList.add(3)
arrayList.add(4)

// It will implicitly covert to Scala collection, 
// so you could use map/foreach...etc.
arrayList.map(_ * 2).foreach(println)

score 8 · Answer 3 · answered Mar 14 '11 at 14:14

8

What's wrong with just

users.foreach(user => println(user.getStatus()))

or even

users.map(_.getStatus()).foreach(println _)

or, if you're worried about traversing the collection twice

users.view.map(_.getStatus()).foreach(println _)

IOW: Why do you want to manage the iteration yourself (and possibly make mistakes), when you can just let someone else do the work for you?

answered Mar 14 '11 at 14:14

Jörg W Mittag

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Nothing's wrong. I've just begun learning Scala. Thanks for the examples! PS: These are Java Lists, will check if your examples work. – Sri Mar 14 '11 at 14:53
Nope, sorry. Twitter4J returns me a java.util.List and .foreach, .map, .view.map don't work out of the box. Is there some neat way to convert these into Scala Lists and make use of .foreach, .map and .view's? – Sri Mar 14 '11 at 14:56
5

@Srirangan: `import collection.JavaConversions._` and it will work. – missingfaktor Mar 14 '11 at 16:12

score 4 · Answer 4 · answered Mar 14 '11 at 16:11

4

I prefer scalaj-collection to scala.collection.JavaConversions. This makes the conversions explicit:

import scalaj.collection.Implicits._

val arrayList = new java.util.ArrayList[Int]
arrayList.add(2)
arrayList.add(3)
arrayList.add(4)

arrayList.asScala.map(_ * 2).foreach(println)

Available here: https://github.com/scalaj/scalaj-collection

answered Mar 14 '11 at 16:11

Kris Nuttycombe

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I didn't know about scalaj-collection, but in Scala 2.8, at least, `import scala.collection.JavaConverters._` will give you explicit `.asScala`conversions if you want them. – Nicolas Payette Mar 14 '11 at 19:02
Yeah, I'd heard about that but haven't had the time to really look into the implementation of JavaConverters. scalaj-collection is pretty clean, so I've kind of stuck to that. – Kris Nuttycombe Mar 14 '11 at 21:27

score 1 · Answer 5 · answered Dec 11 '14 at 11:15

1

I suggest using

scala.collection.JavaConverters._

and simply add .asScala to every object you wish to iterate

answered Dec 11 '14 at 11:15

Ohad Navon

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Unable to iterate Java List in Scala

5 Answers5