Java static class members and Scala interoperability

Question

From the An Overview of the Scala Programming Language, Second Edition:

// Scala
object PrintOptions {
    def main(args: Array[String]): Unit = {
        System.out.println("Options selected:")
        for (val arg <- args)
            if (arg.startsWith("-"))
                System.out.println(" " + arg.substring(1))
    }
}

In the example above, the Scala program invokes methods startsWith and substring of String, which is a class defined in Java. It also accesses the static out field of the Java class System, and invokes its (overloaded) println method. This is possible even though Scala does not have a concept of static class members. In fact, every Java class is seen in Scala as two entities, a class containing all dynamic members and a singleton object, containing all static members.

I understand translation of Scala's companion objects into Java bytecode, but I am not sure what exactly does it means bold text in upper blockquote "is seen in Scala" for opposite example (from Java to Scala).

Does it mean that Java classes with static members are actually converted or just interpreted as two entities in Scala? Or both of my assumptions are wrong?

Daniel C. Sobral Daniel C. Sobral · Accepted Answer · 2012-03-28T15:03:06

I think you might be blinded by Java assumptions. Consider this simple snippet of code:

X.Y()

The means that the method Y is being called on the object X, or on some other object which X was implicitly converted into.

Maybe that doesn't look surprising, or you don't see anything amiss with that, so let's state explicitly a consequence: X will NEVER be a class. You don't invoke methods on classes, period.

Naturally, that presents a serious interoperability problem with Java regarding static members, and that's why it is stated that static members of a Java class X will be "seen" as a singleton object: because, otherwise, you'll never be able to use them.

Note that Scala's singleton objects are true objects -- they are instances of singleton classes. Java classes with static members will not give origin to single objects, though. In practice, this means that this line:

val x = X

will work if X is a Scala singleton object, but won't work if it is a Java class with static members.

Java static class members and Scala interoperability

2 Answers