Welcome to JavaRanch.
Unlike Eclipse, which seems to be happy if you put files into its folders, NetBeans has a UML file with a list of files in use, and seems to require the files be created inside NetBeans. When I have done that I have had to re-create the folder/package structure and use ctrl-c ctrl-v to copy all the code into the .java files. There must be a simpler way to do it . . .
Anybody else?