Question

The Scala compiler compiles direct to Java byte code (or .NET CIL). Some of the features of Scala could be re-done in Java straightforwardly (e.g. simple for comprehensions, classes, translating anonymous/inner functionc etc). What are the features that cannot be translated that way?

That is presumably mostly of academic interest. More usefully, perhaps, what are the key features or idioms of Scala that YOU use that cannot be easily represented in Java?

Are there any the other way about? Things that can be done straightforwardly in Java that have no straightforward equivalent in Scala? Idioms in Java that don’t translate?

Answer 1

This question, in my opinion, misses the point about by asking us to compare JVM languages by looking at their generated bytecode.

Scala compiles to Java-equivalent bytecode. That is, the bytecode could have been generated by code written in Java. Indeed you can even get scalac to output an intermediate form which looks a lot like Java.

All features like traits (via static forwarders), non-local returns (via exceptions), lazy values (via references) etc are all expressible by a Java program, although possibly in a most-ugly manner!

But what makes scala scala and not Java is what scalac can do for you, before the bytecode is generated. What scalac has going for it, as a statically typed language, is the ability to check a program for correctness, including type correctness (according to its type system) at compile time.

The major difference then between Java and scala (as of course Java is also statically typed), therefore, is scala’s type system, which is capable of expressing programmatic relations which java-the-language’s type system cannot.For example:

class Foo[M[_], A](m : M[A])
trait Bar[+A]

These concept, that M is a type parameter which itself has type parameters or that Bar is covariant, just do not exist in Java-land.