javap difference when compiling with/without assertion

Question

Scala for the Impatient Chapter 15 Exercise 10: Add assert(n >= 0 to a factorial method. Compile with assertions enabled and verify that factorial(-1) throws an exception. Compile without assertions. What happens? Use javap to check what happened to the assertion call.

My code:

object Test {
  def factorial(x: Int): Int = {
    assert(x >= 0, "Call to factorial must be >= 0!")
    x match {
      case 0 => 1
      case x: Int => x * factorial(x - 1)
    }
  }

  def main(args: Array[String]): Unit = {
    factorial(-1)
  }
}

I compiled with scalac first, inspected it using javap Test, then compiled again with scalac -Xelide-below MAXIMUM and inspected with the same command - I can't seem to find a difference between the two.

I understand compiling with assertions will throw the exception when I try to execute the program, and compiling without assertions will cause a stack overflow error, but I can't find the difference in javap...

Answer 1

When I try this with javap -v I find the following lines in the version with assertions enabled, but not in the other:

   20:  invokevirtual   #27; //Method scala/Predef$.assert:(ZLscala/Function0;)V
   ...
   27:  if_icmpne       34
   30:  iconst_1
   31:  goto    55

So that certainly looks okay.

The problem may be that you're either not looking at the bytecode (which requires the -c or -v flag to javap), or—much more likely—that you're looking at the output of javap for the Test class, not Test$. See for example Programming in Scala for more detail:

For every Scala singleton object, the compiler will create a Java class for the object with a dollar sign added to the end. For a singleton object named App, the compiler produces a Java class named App$. This class has all the methods and fields of the Scala singleton object.

If you list the contents of the directory you've compiled in, you'll see both Test.class and Test$.class. Using javap -v Test$ will show you the latter, which is where you'll find the difference.