The Bytecode Engineering Library (BCEL, now part of
http://jakarta.apache.org/ ) had an example of a peephole optmizer that
improved the bytecode generated for boolean expressions by early
compilers (although I believe later versions of javac generate improved
code). You may want to have a look at this as it could be easily
extended for other optimisations - in any case I would recommend BCEL
as a useful package for the manipulation of class files.

By the way, do you mind me asking why you want an optimizer? In most
cirumstances a JVM will "JIT" the bytecode to native code, performing
its own optimizations - so optimising the bytecode may not be
beneficial.

I've heard rumours that optimising bytecode could cause a performance
degredation as the JIT might miss opportunities to do its own
optimisation. I hope and suspect this rumour is not true, as it would
imply that a JVM implementation might not optimise code compiled with a
competitor's compiler.

Paul