APK decompilers

JD-Gui

First famous gui Java decompiler, you could use it to investigate the Java code from the APK once you have obtained it.

Jadx

Buildin Java (multi-platform)and at this moment I think it's the recommended one. Just download the latest version and execute it from the bin folder:

jadx-gui

Using the GUI you can perform text search, go to the functions definitions (CTRL + left click on the function) and cross refs (right click --> Find Usage)

If you only want the java code but without using a GUI a very easy way is to use the jadx cli tool:

jadx app.apk

Some interesting options of jadx (GUI and CLI versions) are:

-d <path to output dir>
--no-res #No resources
--no-src #No source code
--no-imports #Always write entire package name (very useful to know where is the function that you might want to hook)

GDA-android-reversing-Tool

Looks faster than JD-Gui, It probides more information (MalScan, Strings...) and same interesting capabilities: X-refs, go to functions definitions... But it's only available for Windows.

Bytecode-Viewer

Another interesting tool to make a Static analysis is: bytecode-viewer. It allows you to decompile the APK using several decompilers at the same time. Then, you can see for example, 2 different Java decompilers and one Smali decompiler. It allows you also to modify the code:

If you modify the code, then you can export it. One bad thing of bytecode-viewer is that it doesn't have references or cross-references.