...When you runed the Decompiler than created an folder with all Files from then APK and the dex (dalvik executable format) being converted to an jar file. You must only now use an Java Decompiler like jd-gui and you have all files of the App.
Decouple your GUI building code from the rest of your application. Using an XML description, the JavaGui Builder will build appropriate windows, controls and objects for later retrieval by the mainstream code.