Android is meant to be open source. And most components, despite being covered by the Apache license, have publicly available source code. Unfortunately, the sad truth is that only Nexus devices owners can do Java modifications without digging into Smali assembler language, which is not simple and needs much more effort than Java. Also, decompiled applications can’t be imported to Eclipse or Android Studio.
There are some tools like GetJava that already can do the job, but in most situations the result isn’t 100% accurate and some files still need to be translated to Java. XDA Senior Member darkguy2008 decided to start a project aimed to provide a better solution than JAD or JD-GUI.
The project is still at a very early stage, but most things are working already. This project is written in C# and needs Visual Studio 2012 and .NET Framework 4.5 installed to work properly. Hopefully in the future, it will be possible to use it on other operating systems like Linux or Mac OS X. Undoubtedly, this project has terrific potential and with help of other developers, the Android development can be significantly improved.
More information regarding this converter can be found in the original thread, so don’t hesitate to go there and give some input to the developer. Of course, you can also contribute by pushing some patches to the Github repository.
Bear in mind that tools like this should not be used to get some freebies from paid apps and re-release it under your name. Developers sell their work for a reason, so you should use it only for educational purposes.