Android Studio 3.2 Generated java

This folder has already been in your project, only now it is highlighted in the Android view mode of the project explorer. Its location is actually build/generated/source/r/<current flavor>/<current build type>/.

The classes you can find here are all the classes the project will be built from, including the libraries. Inspecting these could be useful when you are having issues with generated code, such as annotation processors. For example, the databinding library, ButterKnife, or Dagger 2 all generate java classes based on your annotations, which you can see in this directory.

If you delete the directory, it will make the next build slower, because it needs to regenerate these files again, instead of reusing them. Which means they will reappear anyways, so there's no point in doing that.