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 class. I would like to use the iterator in a for each style loop, so I have been trying to convert it to a native Scala collection but will no luck.

I have looked at the Scala wrapper classes/traits, but I can not see how to use them correctly. How do I iterate over a Java collection in Scala without using the verbose while(hasNext()) getNext() style of loop?

Here's the code I wrote based on the correct answer:

class IteratorWrapper[A](iter:java.util.Iterator[A]) { def foreach(f: A => Unit): Unit = { while(iter.hasNext){ f(iter.next) } } } object SpreadsheetParser extends Application { implicit def iteratorToWrapper[T](iter:java.util.Iterator[T]):IteratorWrapper[T] = new IteratorWrapper[T](iter) override def main(args:Array[String]):Unit = { val ios = new FileInputStream("assets/data.xls") val workbook = new HSSFWorkbook(ios) var sheet = workbook.getSheetAt(0) var rows = sheet.rowIterator() for (val row <- rows){ println(row) } } }

I can't seem to include the line "for (val row <- rows){" without the parser thinking the '<' character is an XML closing tag? The backticks do not work — BefittingTheorem
– BefittingTheorem, Commented Jan 30, 2009 at 17:15
You should be able to convert to IteratirWrapper implicitly, saving you a fair bit of syntax. Google for implicit conversions in Scala. — Daniel Spiewak
– Daniel Spiewak, Commented Jan 30, 2009 at 18:01

2 revs, 2 users 83% · Accepted Answer · 2013-06-09 20:18:35Z

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.

2 revs · Accepted Answer · 2019-08-21 07:53:52Z

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)

4 revs, 4 users 67% James · Accepted Answer · 2014-07-06 01:15:51Z

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.

It should read: scala.collection.jcl.MutableIterator.Wrapper
This answer is obsolete in Scala 2.8; see stackoverflow.com/questions/2708990/…

Peter Mortensen · Accepted Answer · 2013-06-09 20:16:10Z

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.

2 revs, 2 users 80% · Accepted Answer · 2013-06-09 20:18:52Z

For Scala 2.10:

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

Max · Accepted Answer · 2016-01-01 00:43:51Z

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 :)

Fabian Steeg · Accepted Answer · 2009-01-30 15:58:31Z

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)

user4322779 · Accepted Answer · 2015-05-16 20:09:09Z

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 } }

kilo · Accepted Answer · 2015-08-07 06:59:28Z

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.

I used import scala.collection.JavaConverters._ and then javaList.iterator().asScala from your example and it worked

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