You want to know how to reverse-engineer an android classes file? This approach may work, if you want to reverse unencrypted files.
1. Rename your .apk to .zip
2. Unzip
3. locate classes.dex
4. Download Dex2Jar (Windows/Linux)
5. Use cmd/shell to navigate to dex2jar Folder.
6. Use command "dex2jar <source>" with source as your classes.dex file.
7. Now you have a new .jar file. This is the output of the .dex file.
8. You can use JDGUI to decompile the binaries and look at the java code.
Its pretty ease, you just have to move the .jar into the jd-gui application.