From the mailing list:

have to make sure my code runs on Java 5, therefore I'm developing
with Java 5 on the classpath of my project. I've installed Java 5 on
Snow Leopard following these instructions:

http://wiki.oneswarm.org/index.php/OS%5FX%5F10.6%5FSnow%5FLeopard

This makes sure I have a real Java 5 SDK and not just a symlink that
points to the Java 6 SDK. The point being, of course, that the
compiler should make sure I'm not using any Java 6 API in my code.
But when I try this, which should fail because the String constructor
that takes a byte array and a Charset is new in Java 6, it does
compile:

object Test { 
  def main(args: Array[String]) { 
    val b = Array[Byte](34, 35, 36) 
    val s = new String(b, Charset.forName("UTF-8")) 
  } 
}

and fails at runtime with a java.lang.NoSuchMethodError:
java.lang.String.<init>([BLjava/nio/charset/Charset;)V
A Java file in the same project (hence same classpath) correctly
refuses to build the corresponding Java code with error "The
constructor String(byte[], Charset) is undefined".