1. unzip program.apk classes.dex
2. Download [https://github.com/pxb1988/dex2jar](https://github.com/pxb1988/dex2jar)
3. Run `dex2jar/dex2jar.sh ./classes.dex`
4. Download [http://jd.benow.ca/](http://jd.benow.ca/)
5. Run `./jd-gui classes.dex.dex2jar.jar`

Ref: http://pof.eslack.org/2011/02/18/from-apk-to-readable-java-source-code-in-3-easy-steps/