How can I see in what [Java/Scala?] code does Scala compiler rewrites original Scala-code

Question

Following Scala mailing lists, different people often say: "compiler rewrites this [scala] code into this [java/scala??] code". For example, from one of the latest threads, if Scala sees

class C(i: Int = 4) { ... }

then the compiler rewrites this as (effectively):

class C(i: Int) { ... }
object C {
  def init$default$1: Int = 4
}

How can I find out, what will be the compiler output for my code? Should I decompile the resulting bytecode for that?

Answer 1

You can use "-print" as compiler option, and scalac will remove all Scala-specific features.

For example, here is the original code:

class Main
{
    def test (x: Any) = x match {
        case "Hello" => println ("Hello World")
        case e: String => println ("String")
        case i: Int => println ("Int")
        case _ => println ("Something else")
    }
}

And if you use "scalac -print" to compile it, you will get the following Scala code.

[[syntax trees at end of cleanup]]// Scala source: Test.scala
package <empty> {
  class Main extends java.lang.Object with ScalaObject {
    def test(x: java.lang.Object): Unit = {
      <synthetic> val temp1: java.lang.Object = x;
      if (temp1.==("Hello"))
        {
          scala.this.Predef.println("Hello World")
        }
      else
        if (temp1.$isInstanceOf[java.lang.String]())
          {
            scala.this.Predef.println("String")
          }
        else
          if (temp1.$isInstanceOf[Int]())
            {
              scala.this.Predef.println("Int")
            }
          else
            {
              scala.this.Predef.println("Something else")
            }
    };
    def this(): Main = {
      Main.super.this();
      ()
    }
  }
}

Answer 2

One can look at the generated bytecode with

javap -c -private ClassNameWithoutDotClass