Check out example codes for "java decompiler". It will help you in understanding the concepts better.

Code Example 1

Tools for mac:
1- http://www.javadecompilers.com/
2- http://java-decompiler.github.io/

In terminal:
add permitions
chmod u+x d2j_invoke.sh
chmod u+x d2j-dex2jar.sh

Create java class in terminal
sh d2j-dex2jar.sh classes.dex

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