score:29

Accepted answer

There is a wrapper class (scala.collection.jcl.MutableIterator.Wrapper). So if you define

implicit def javaIteratorToScalaIterator[A](it : java.util.Iterator[A]) = new Wrapper(it)

then it will act as a sub class of the Scala iterator so you can do foreach.

score:2

If you would like to avoid the implicits in scala.collection.JavaConversions you can use scala.collection.JavaConverters to convert explicitly.

scala> val l = new java.util.LinkedList[Int]()
l: java.util.LinkedList[Int] = []

scala> (1 to 10).foreach(l.add(_))

scala> val i = l.iterator
i: java.util.Iterator[Int] = java.util.LinkedList$ListItr@11eadcba

scala> import scala.collection.JavaConverters._
import scala.collection.JavaConverters._

scala> i.asScala.mkString
res10: String = 12345678910

Note the use of the asScala method to convert the Java Iterator to a Scala Iterator.

The JavaConverters have been available since Scala 2.8.1.

score:4

You could convert the Java collection to an array and use that:

val array = java.util.Arrays.asList("one","two","three").toArray
array.foreach(println)

Or go on and convert the array to a Scala list:

val list = List.fromArray(array)

score:5

With Scala 2.10.4+ (and possibly earlier) it is possible to implicitly convert java.util.Iterator[A] to scala.collection.Iterator[A] by importing scala.collection.JavaConversions.asScalaIterator. Here is an example:

object SpreadSheetParser2 extends App {

  import org.apache.poi.hssf.usermodel.HSSFWorkbook
  import java.io.FileInputStream
  import scala.collection.JavaConversions.asScalaIterator

  val ios = new FileInputStream("data.xls")
  val workbook = new HSSFWorkbook(ios)
  var sheet = workbook.getSheetAt(0)
  val rows = sheet.rowIterator()

  for (row <- rows) {
    val cells = row.cellIterator()
    for (cell <- cells) {
      print(cell + ",")
    }
    println
  }

}

score:6

If you are iterating through a large dataset, then you probably don't want to load whole collection into memory with .asScala implicit conversion. In this case, a handy way approach is to implement scala.collection.Iterator trait

import java.util.{Iterator => JIterator}

def scalaIterator[T](it: JIterator[T]) = new Iterator[T] {
  override def hasNext = it.hasNext
  override def next() = it.next()
} 

val jIterator: Iterator[String] = ... // iterating over a large dataset
scalaIterator(jIterator).take(2).map(_.length).foreach(println)  // only first 2 elements are loaded to memory

It has similar concept but less verbose IMO :)

score:9

For Scala 2.10:

// Feature warning if you don't enable implicit conversions...
import scala.language.implicitConversions
import scala.collection.convert.WrapAsScala.enumerationAsScalaIterator

score:15

The correct answer here is to define an implicit conversion from Java's Iterator to some custom type. This type should implement a foreach method which delegates to the underlying Iterator. This will allow you to use a Scala for-loop with any Java Iterator.

score:35

Edit: Scala 2.13.0 deprecates scala.collection.JavaConverters, so since 2.13.0 you need to use scala.jdk.CollectionConverters.

Scala 2.12.0 deprecates scala.collection.JavaConversions, so since 2.12.0 one way of doing this would be something like:

import scala.collection.JavaConverters._

// ...

for(k <- javaCollection.asScala) {
    // ...
}

(notice the import, new is JavaConverters, deprecated is JavaConversions)

score:258

As of Scala 2.8, all you have to do is to import the JavaConversions object, which already declares the appropriate conversions.

import scala.collection.JavaConversions._

This won't work in previous versions though.


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