The Android platform cames with a handy disassembler tool, called “dexdump”. You find this in your Android SDK installation directory in the folder”platform-tools”.
This allows you to create the Dalvik Virtual Machine bytecode from a dex file. If you are using Eclipse you can for example use the Navigator view to see the “bin” directory. This directory contains the file “classes.dex”. This is a dalvik executable generated from the compiled classes. To see the bytecode of this file, run:
dexdump -d ~vogella/Desktop/classes.dex | less
This will show you the Dalvik Bytecode. To compile the Dalvik Bytecode to Java source code there are no official tools. You can use http://code.google.com/p/dex2jar/ to compile to jar files and continue your path from there. Thanks to Bernhard Merkle for the tip with dex2jar.