Auto-code generation is a very powerful tool that every software engineer should use. It avoids coding that tedious and repetitive boilerplate that nobody should ever have to write, primarily because it is usually uninteresting code and highly prone to error. This talk will briefly show the annotation processor. We will understand what are the annotation processors and their defined role and then focus on how to get started writing your own annotation processor to generate source code automatically, as well as how popular frameworks such as Dagger, Butterknife or such work in the background.