After a program has been thrown into the world in binary form, it can boomerang back as source code. The Boomerang reverse engineering framework is the first general native executable decompiler available to the public.
jdec is a Java Decompiler. It can be used to reverse the bytecodes present in a .class File to produce a Java Source File which can be nearly correct or equivalent (Due To different Interpretations) to the original java file.[With UI & command line]