Conversions Between Java and Scala Collections

Like Scala, Java also has a rich collections library. There are many similarities between the two. For instance, both libraries know iterators, iterables, sets, maps, and sequences. But there are also important differences. In particular, the Scala libraries put much more emphasis on immutable collections, and provide many more operations that transform a collection into a new one.

Sometimes you might need to pass from one collection framework to the other. For instance, you might want to access an existing Java collection as if it were a Scala collection. Or you might want to pass one of Scala’s collections to a Java method that expects its Java counterpart. It is quite easy to do this, because Scala offers implicit conversions between all the major collection types in the JavaConverters object. In particular, you will find bidirectional conversions between the following types.

Iterator               <=>     java.util.Iterator
Iterator               <=>     java.util.Enumeration
Iterable               <=>     java.lang.Iterable
Iterable               <=>     java.util.Collection
mutable.Buffer         <=>     java.util.List
mutable.Set            <=>     java.util.Set
mutable.Map            <=>     java.util.Map
mutable.ConcurrentMap  <=>     java.util.concurrent.ConcurrentMap

To enable these conversions, simply import them from the JavaConverters object:

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

This enables conversions between Scala collections and their corresponding Java collections by way of extension methods called asScala and asJava:

scala> import collection.mutable._
import collection.mutable._

scala> val jul: java.util.List[Int] = ArrayBuffer(1, 2, 3).asJava
jul: java.util.List[Int] = [1, 2, 3]

scala> val buf: Seq[Int] = jul.asScala
buf: scala.collection.mutable.Seq[Int] = ArrayBuffer(1, 2, 3)

scala> val m: java.util.Map[String, Int] = HashMap("abc" -> 1, "hello" -> 2).asJava
m: java.util.Map[String,Int] = {abc=1, hello=2}

Internally, these conversions work by setting up a “wrapper” object that forwards all operations to the underlying collection object. So collections are never copied when converting between Java and Scala. An interesting property is that if you do a round-trip conversion from, say a Java type to its corresponding Scala type, and back to the same Java type, you end up with the identical collection object you have started with.

Certain other Scala collections can also be converted to Java, but do not have a conversion back to the original Scala type:

Seq           =>    java.util.List
mutable.Seq   =>    java.util.List
Set           =>    java.util.Set
Map           =>    java.util.Map

Because Java does not distinguish between mutable and immutable collections in their type, a conversion from, say, scala.immutable.List will yield a java.util.List, where all mutation operations throw an “UnsupportedOperationException”. Here’s an example:

scala> val jul = List(1, 2, 3).asJava
jul: java.util.List[Int] = [1, 2, 3]

scala> jul.add(7)
java.lang.UnsupportedOperationException
  at java.util.AbstractList.add(AbstractList.java:148)