Decompilers online

Until recently, you needed to use a Java decompiler and all of them were either unstable, obsolete, unfinished, or in the best case all of the above. And, if not, then they were commercial. The obsoleteness was typically proved by the fact that they can only decompile JDK 1.3 bytecode.
The only so-so working solution was to take the .class file and pre-process it, so it becomes JDK 1.3 compatible, and then run Jad over it (one of those older, but better decompilers).
But recently, a new wave of decompilers has forayed onto the market: Procyon, CFR, JD, Fernflower, Krakatau, Candle.

Android application package (APK) is the package file format used to distribute and install application software onto Google's Android operating system. This site uses a perfect open-source APK and DEX decompiler called Jadx. Jadx decompiles .class and .jar files, but also it produces Java source code from Android Dex and Apk files.

The resources in an Adroid APK file are compressed and stored in binary format.
Apktool is an instrument for reverse engineering 3rd party, closed, binary Android apps. It can decode resources to its original form.
You may use this website to extract and research assets and the manifest file.