Iterating over Java collections in Scala

Question

I\'m writing some Scala code which uses the Apache POI API. I would like to iterate over the rows contained in the java.util.Iterator that I get from the Sheet

Answer 1

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.

Answer 2

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)

Answer 3

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.

Answer 4

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
  }

}

Answer 5

For Scala 2.10:

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

Answer 6

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)