I haven't looked, but even if the bytecodes haven't changed (and I
suspect they haven't), there's a danger that a builtin class in 1.5 may
not exist in 1.4 or may not have all the fields or methods that 1.5 has.

--Mike Amling

All the new semantics is realized through tricks that work just as well on
pre-1.5 JVMs.  A few new attributes have been added, but that's completely
downwards compatible.

However, from a post by Neil Grafter on c.l.j.programmer:

   You can translate 1.5-style classfiles into 1.4-style classfiles
   by discarding the stackmaps and changing the version number.
   Almost.  Class literals generate a single bytecode instruction
   (ldc) in 1.5, and that must be translated into the mass of code
   that was used in previous releases.

So I suspect that (if the feature makes it into the final release) code
compiled with -target 1.5 may not be able to run on a 1.4 JVM (but then, I
don't suppose it would anyway given the version number checking).

   -- chris