Using `Set` Argument to Scala Code in Java

Question

I'm trying to call my Scala Util object from Java code:

Main.java

Set<Long> items = new HashSet<Long>();
// fill up items with Long
MyUtil.foo(100, items);

Util.scala

object Foo {
 type Id = Long
 def foo(id: Id, items: scala.collection.mutable.Set[Id])

Here's the compile-time error:

  could not parse error message:   
  required: long,scala.collection.mutable.Set<Object>
  found: Long,java.util.Set<Long>
  reason: actual argument java.util.Set<Long> cannot be converted to 
      scala.collection.mutable.Set<Object> by method invocation conversion`

From reading these Java to Scala Collections docs, I am using a mutable Set rather than the default, immutable Set:

scala.collection.mutable.Set <=> java.util.Set

But, I don't understand the error message. By using a Long (boxed long) in my Java code, why is a Set<Long> found?

Those conversion work as far as I understand while in .scala. Java can not cast from class A to B like that. — zapl
– zapl, Commented Jan 30, 2014 at 20:55

som-snytt · Accepted Answer · 2014-01-30 21:35:43Z

2

Demonstrating what the commenter said:

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

scala> val js = (1 to 10).toSet.asJava
js: java.util.Set[Int] = [5, 10, 1, 6, 9, 2, 7, 3, 8, 4]

scala> def f(is: collection.mutable.Set[Int]) = is.size
f: (is: scala.collection.mutable.Set[Int])Int

scala> def g(js: java.util.Set[Int]) = f(js.asScala)
g: (js: java.util.Set[Int])Int

scala> g(js)
res0: Int = 10

answered Jan 30, 2014 at 21:35

som-snytt

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3 Comments

Kevin Meredith Over a year ago

Hi @som-snytt. Would error: value toSet is not a member of java.util.Set[Long] show up here: foo(items: java.util.Set[Long]) { val scalaSet = items.toSet } due to the java.util.Set type?

som-snytt Over a year ago

Yes. So the preferred idiom is to use JavaConverters (not Conversions), and call asJava and asScala to convert, as shown. Then in your method that takes a Java set, call asScala on it first. You could also call js.asScala.toSet to build an immutable Set.

Kevin Meredith Over a year ago

I ran into another error that may or may not be related to this one - stackoverflow.com/questions/21483121/…. If you get time, please take a look. Thanks

Community · Accepted Answer · 2017-05-23 12:28:38Z

1

Working with Scala collections and type aliases from Java code (not the converse, as som-snytt showed :) is going to be at least nasty, most likely very painful, and quite possibly impossible.

If you're able to modify the Scala side of the API, I'd recommend adding a Java-friendlier API to it. If not, I guess you could build an adapter layer in Scala that proxies Java clients through to the native Scala API.

So, something like:

// Original Scala
object Foo {
  type Id = Long
  def foo(id: Id, items: scala.collection.mutable.Set[Id]) 
}

// Java adapter -- generics might be made to work on the Java side, 
// but Long is particularly problematic, so we'll just force it here
object FooJ {
  import scala.collection.JavaConverters._

  def foo(id: Long, items: java.util.Set[Long]) = {
    Foo.foo(id, items.asScala)
  }
}

edited May 23, 2017 at 12:28

CommunityBot

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answered Jan 30, 2014 at 21:42

Alex Cruise

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2 Comments

Kevin Meredith Over a year ago

Thanks. Perhaps it's obvious, but ju.Set[Long] is an abbreviation for java.util.Set[Long]?

Alex Cruise Over a year ago

Whoops, yeah. Fixed. :)

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