java.util.Iterator to Scala list?

Question

I have the following code:

private lazy val keys: List[String] = obj.getKeys().asScala.toList

obj.getKeys returns a java.util.Iterator<java.lang.String>

Calling asScala, via JavaConverers (which is imported) according to the docs..

java.util.Iterator <==> scala.collection.Iterator

scala.collection.Iterator defines

def toList: List[A]

So based on this I believed this should work, however here is the compilation error:

[scalac]  <file>.scala:11: error: type mismatch;
[scalac]  found   : List[?0] where type ?0
[scalac]  required: List[String]
[scalac]  private lazy val keys : List[String] = obj.getKeys().asScala.toList
[scalac]  one error found

I understand the type parameter or the java Iterator is a Java String, and that I am trying to create a list of Scala strings, but (perhaps naively) thought that there would be an implicit conversion.

Matthew Farwell · Accepted Answer · 2011-10-01 23:07:25Z

27

You don't need to call asScala, it is an implicit conversion:

import scala.collection.JavaConversions._

val javaList = new java.util.LinkedList[String]() // as an example

val scalaList = javaList.iterator.toList

If you really don't have the type parameter of the iterator, just cast it to the correct type:

javaList.iterator.asInstanceOf[java.util.Iterator[String]].toList

EDIT: Some people prefer not to use the implicit conversions in JavaConversions, but use the asScala/asJava decorators in JavaConverters to make the conversions more explicit.

edited Oct 1, 2011 at 23:07

answered Sep 28, 2011 at 21:19

Matthew Farwell

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3 Comments

rshepherd Over a year ago

You are right, there is no type parameter for the Iterator that is being returned from the getKeys() method. Very astute observation, thank you. Though it appears I do need the asScala in there, in that case. Perhaps there is something else I am missing.

Daniel C. Sobral Over a year ago

The implicit conversion is on JavaConversions, but not on JavaConverters. That uses asScala. Many people, myself included, prefer to make this conversion explicit to avoid subtle bugs.

Matthew Farwell Over a year ago

@Daniel I guess making stuff explicit is a good idea, thanks for the tip.

Didier Dupont · Accepted Answer · 2011-09-28 21:39:42Z

11

That would work if obj.getKeys() was a java.util.Iterator<String>. I suppose it is not.

If obj.getKeys() is just java.util.Iterator in raw form, not java.util.Iterator<String>, not even java.util.Iterator<?>, this is something scala tend to dislikes, but anyway, there is no way scala will type your expression as List[String] if it has no guarantee obj.getKeys() contains String.

If you know your iterator is on Strings, but the type does not say so, you may cast :

obj.getKeys().asInstanceOf[java.util.Iterator[String]]

(then go on with .asScala.toList)

Note that, just as in java and because of type erasure, that cast will not be checked (you will get a warning). If you want to check immediately that you have Strings, you may rather do

obj.getKeys().map(_.asInstanceOf[String])

which will check the type of each element while you iterate to build the list

answered Sep 28, 2011 at 21:39

Didier Dupont

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1 Comment

rshepherd Over a year ago

Thanks very much for the answer! seems like you and Matthew Farwell agree for the most part. I did what you suggest, obj.getKeys().asInstanceOf[java.util.Iterator[String]].asScala.toList and that seems to work, and I did not get a compilation warning.

Daniel C. Sobral · Accepted Answer · 2011-10-01 23:02:36Z

3

I dislike the other answers. Hell, I dislike anything that suggests using asInstanceOf unless there's no alternative. In this case, there is. If you do this:

private lazy val keys : List[String] = obj.getKeys().asScala.collect { 
    case s: String => s 
}.toList

You turn the Iterator[_] into a Iterator[String] safely and efficiently.

answered Oct 1, 2011 at 23:02

Daniel C. Sobral

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Comments

Xavier Guihot · Accepted Answer · 2019-06-08 23:01:53Z

2

Note that starting Scala 2.13, package scala.jdk.CollectionConverters replaces deprecated packages scala.collection.JavaConverters/JavaConversions when it comes to implicit conversions between Java and Scala collections:

import scala.jdk.CollectionConverters._

// val javaIterator: java.util.Iterator[String] = java.util.Arrays.asList("a", "b").iterator
javaIterator.asScala
// Iterator[String] = <iterator>

edited Jun 8, 2019 at 23:01

answered Mar 31, 2019 at 21:12

Xavier Guihot

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Andriy · Accepted Answer · 2018-12-12 16:34:55Z

0

As of scala 2.12.8 one could use

import scala.collection.JavaConverters._

asScalaIterator(java.util.Iterator variable).toSeq

answered Dec 12, 2018 at 16:34

Andriy

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