Java interop: no more static 'empty' for collections

[raboof]

scala/scala@a864ae0 (added between 2.13.0-M5 and 2.13.0-RC1) introduces a def empty() on scala.collection.Iterable.

Many (immutable) implementations of Iterable, such as scala.collection.immutable.List, have a companion object with a val empty. On 2.13.0-M5 and earlier, Java users could use the List.empty() static forwarder to get the empty List instance. From 2.13.0-RC1, this static forwarder no longer exists, because the List class itself now has a non-static empty() method inherited from Iterable.

According to the commit message the change was intended to "facilitate the switch to dotty's for comprehensions" - I don't know anything about that, so it might well be that the change is inevitable or worth it for that reason - I just wanted to flag this consequence that might not have been intentional.

What would be the new way for a Java user to get easy access to an empty instance of such an immutable collection?

[hrhino]

asScalaIterable(Collections.newArrayList()).toList(), of course. (They're used to the verbosity.)

We could slap @varargs on List.apply and friends, but then we'd have to fix #10683. (I did a bit of work on that some moons ago; it's not trivial but may be Java-interop-specific to be considered for 2.13).

That would also give me List.apply("foo", "bar"), which I've been missing since forever.

[szeiger]

I'm not familiar with changes in for comprehensions in Dotty so I'll leave that to @adriaanm. But if they require this method then the change is probably inevitable.

You can always get the "static" parts in Java via MODULE$: https://github.com/scala/scala/blob/2.13.x/test/files/pos/t1642/JavaCallingScalaHashMap.java

[hrhino]

I think the change hasn't happened yet (or I didn't notice) but it's in pursuit of scala/scala3#2573 as I understand it. scala/scala-dev#607 mentions that we could wait for 2.14, but in scala/scala#7929 @adriaanm expressed a desire to make no backwards-incompatible changes in 2.14.

So depending on Dotty's willingness to run off of 2.14.x, (or merge scala/scala3#5813), maybe this could just be backed out. (Unless the for-comprehension improvement is already merged?)

[szeiger]

Scheduling for RC2 in case we want to revert the empty() change.

[smarter]

Or add a static List.of (like https://docs.oracle.com/javase/9/docs/api/java/util/List.html#of-E...-) for the express purpose of Java interop ?

[smarter]

Also to clarify, dotty doesn't care about empty being there or not currently, but I'd love it if scala/scala3#2573 got solved in one way or another.

[hrhino]

of! I had forgotten about that in Java 9.

I like it, but does that mean we'd have two ways of creating the same List? Might be confusing.

[smarter]

evil suggestion: make it private[scala] so that it's public from Java's POV but not Scala's :).

[lrytz]

Affects also play (scalacommunitybuild/playframework@80657d0), akka-http (scalacommunitybuild/akka-http@6d03ee4)

[adriaanm]

I think it's quite likely an improved for-comprehension desugaring would avoid filter/withFilter and use empty instead, so I'd prefer to keep the method in place. We could provide some convenience methods for Java to create Scala collections outside of the stdlib.