What's missing seems to be the directory tree which should reflect the package names.
In short: look at the contents of the original LS_StockListDemo_DataAdapter.jar in the "lib" folder (you can open it as a zip file): it contains a directory tree and the .class files are under internal directories;
you should recreate the same directory tree under the "classes" folder.

We haven't pointed out this requirement in our documentation because this is a java convention; the provided source files also follow this convention.
You don't need to create the directory tree from scratch, as it should have already been created by your compiler upon compilation of your modified sources.