Is there a way to extract the source code from the APK file?

Simple method: Using the online tool http://www.javadecompilers.com/apk, upload apk file and extract source code.

This is the process of decoding the apk file. Follow me in order.

Open a new folder.

There's nothing special about it, and all you have to do is copy the two created folders (class / xml) into one folder. Congratulations.