Sure. Here's some code that handles different situations (no data directory, no files in data directory, files in directory).

Also, all the files names are converted to strings. So you can use those in your program as well.

Code Example

1. File dir = new File(sketchPath + "/data");
2. String[] files = dir.list();
3.  
4. if (files == null) {
5.   println("No data directory");
6. } else {
7.   if (files.length > 0) {
8.     for (int i=0; i<files.length; i++) {
9.       String filename = files[i];
10.       println(files[i]);
11.     }
12.   } else {
13.     println("No files in the data directory");
14.   }
15. }

Thankyou guys,
i'll try these solutions that seem to work,
thought it seems like "File" is not a reserved word for Processing (e.g. it doesn't change color)...

How does it work?

File is not a reserved word for Processing, it is part of the greater Java.

In my example, fileName is... a file name, of the file we want to check if it exists in the data folder.

ok, but dataFile is just before fileName.

I mean: we have a photo.tif, loaded as String from a file.txt,
is dataFile kind of image, or maybe String? or what else?

Uuuh, you never saw a function call? I suggest to start by reading the Processing tutorials...

Seriously, dataFile() is just a function (I prefer function names that start with a verb, showing better they are functions), but it isn't documented in the Language Reference, only in the code's JavaDoc.
And as shown, dataFile() returns a File out of the given String.