Issue 1: Importing from the default package
The program I was writing was actually a homework assignment for a MOOC. It involved a single Java class, and I was writing it in the NetBeans IDE (which will prove germane). I had earlier taken another MOOC (Algorithms, Part I, taught by Robert Sedgewick and Kevin Wayne of my alma mater, Princeton University -- highly recommended!) and downloaded a pair of Java libraries associated with their book (Algorithms, 4th ed.). The libraries are licensed under the GPLv3, and I wanted to use a convenience class from one in the new program. So I added it to the library list in my NetBeans project and serenely tried to import the class I wanted. And failed. Miserably.
After various unproductive dinking around (and swearing), I realized that the problem was that my new Java class was in a named package but the class I was importing was in the library's default package. Using named packages is the norm in Java -- NetBeans will issue a warning if you try to park a class in the default package -- but for reasons related to grading in their MOOC, Sedgewick and Wayne had put all their classes in the default package (and required classes written by students for homework to be in the default package). It turns out that Java will not let a class in any other package import from the default package. Classes in the default package can import from each other and from classes in named packages. I guess this is a security matter. Sedgewick and Wayne provide alternative versions of their libraries using named packages, and point out this very issue in the "Q & A" section of their download page, which I had read and immediately forgotten. Hence this note to myself (since I will inevitably forget again, in some other context).
Issue 2: "Could not find or load main class ..."
Once I resolved the aforementioned problem, I thought I was good to go -- no compilation errors -- and so I tried to run my program (emphasis on the word "tried"). NetBeans announce that it could not find or load my main class. The message is a trifle unhelpful, as it does not specify whether the issue is that the main class could not be found or that it could not be loaded.
Since I selected the main class from NetBeans's run configuration menu, it seemed apparent to me that the class could be found, and for the life of me I had no idea why it could not be loaded. So I was off on a Google search that found a fair number of (somewhat dated) pages with people asking for help and specifying the same error message. Unfortunately, none of the specifics were germane to me, and I did not find any responses that helped.
Eventually I correctly guessed this issue, and I'll record it here in case anyone else is searching for that error message. The problem in my case was the path to the NetBeans project. Recall that the program I was writing was for another MOOC. I had named the parent folder with the full name of the MOOC, including spaces and a colon (which I escaped). My operating system (Linux Mint) has no problem with the presence of the colon, but apparently NetBeans (or Ant, which NetBeans uses to process build scripts) did. As a veteran of DOS and survivor of early versions of Windows, I cut my teeth on 8.3 file names (and directory names), so I suppose I should know better than to get creative with folder names. Renaming the parent folder without any punctuation marks solved that problem.
Not an issue: Regex
This last item is not a problem, it's just a neat web site I found (in an answer on Quora) whose link I wish to preserve. Online regex tester and debugger provides a web application that lets you test regular expressions to see if they do what you think/want them to do. Very useful -- which I'd seen it sooner!