I saw this message occasionally in the past.
It is indeed to ensure sketches are working correctly on all kinds of platforms, some being case sensitive.

" my program relies on typed user input to open files"
As you noted, this is a fragile way to select a file, as users can make other typos than case mess up.
Perhaps you can use selectInput() instead?

Otherwise, a way to workaround the problem is to use File.list() to get a list of files, then loop over this list and see if the lowercase version of a listed file matches the lowercase version of the inputted name.

You can convert the name to lower case. Compare the name to list of existing files (with some leniency in terms of capitals) and use the actual filename based on a soft match. Wouldn't it be safer anyway to let the user choose from a list of existing files? Otherwise, there is a great chance that the typed file won't exist (regardless of capitals).

Oh, I agree that this isn't exactly a practical thing - I'm making a prototype for testing a specific use case with users, not building something I would every put out there.

The idea is that it partially uses text input like a CLI - which means the user must be able to look up the name of a file and open it with text input.

Anyway, I went with putting all strings through toLowerCase() before putting them in, and making sure the "sandbox" only has lowercase files.

@amnon.owed: I first tried something similar:

1. boolean fileExists(String txt) {
2.   File f = new File(sketchPath + "/data", txt);
3.   return f.exists();
4. }

and then do "if (fileExists(input))" - but this turned out to return true for files that don't match in capitalization (so how does Processing tell the difference then?).

EDIT: Also, why was this moved from programming questions to general/other? This is a programming question isn't it?

@Move: Arbitrary maybe, but it makes sense. To be clear (because text without human expression tends not to be) I was merely curious, not complaining.

Also, thank you for the code, I'll modify it to catch filename mismatches before Processing complains.