Scala: Difference between file.class and file$.class from scalac

Question

When I use scalac to compile file.scala, I end up with 2 outputs, file.class and file$.class. What is the difference between these files and which is the appropriate one to then run? I get distinctly different error messages between executing "scala file" vs "scala file$".

Answer 1

Scala objects get compiled to classes ending in "$" because you're allowed to have an "ordinary" class with the same name. But the object's methods are also exposed as static methods on the "ordinary" class, so that they can be called under the names you would expect. This is an artifact of trying to represent the scala semantics in a way that make sense to Java / the JVM, and I would encourage you to regard it as an implementation detail rather than something important.

(@MattPutnam's answer is correct that anonymous classes, including closures, are compiled to class files with $es in their name, but that's not what's causing your file$.class in this particular instance)

Use scala file. If you're interested in the implementation details you might also want to try java -cp /path/to/scala-library.jar file.

Answer 2

file$.class is some inner anonymous class. In Java they're very explicit, but they can be easy to miss in Scala. If you use any method that takes a function, there's an implicit anonymous class there. Post the code and I'll point it out.