There's a better way:

Most classes need to be instantiated, that is, make an object copy of it, before using any of its resources!
The only exception is when some field or method from a class is static.
However, list() isn't a static method of class File! Thus, if you wanna use it, you ought to instantiate it 1st!

final String DIRECTORY = "./"; // <-- choose your path
final File fileInstance = new File(DIRECTORY); // instantiate class File

final String[] test = fileInstance.list(); // now list() from File can be used!

println(test);
exit();

You can also use:

1. println(new File(sketchPath).list());

You only have to remember that classes can contain methods. To call a class, you have to use NEW.

To call a class, you have to use new.

That is only appropriate if it's just one-use shot!
If more than 1 access is needed later on, it's advisable to formally instantiate it once,
using a variable to store its reference pointer (memory address)!

Keyword new allocates enough memory to turn a whole class into an instantiated object!
Doing so all the time is both a waste of performance and memory.

And later on, Java's GC will have more orphaned references to clean up.
And that's even more performance penalty on top of it!

P.S.: Just remember that any method or field from a class which is static,
can be directly accessed w/o any need to instantiate it 1st! 

You might try this. As the name suggests it returns files with a .txt extension that are in the sketch folder

1. import java.io.*;
2. 
   
3. public void setup() {
4.   File[] textfiles1 = listTextFiles(new File(sketchPath));
5. }
6. 
   
7. public File[] listTextFiles(File folder) {
8.   return folder.listFiles(new FilenameFilter() {
9.     public boolean accept(File dir, String name) {
10.       return (!name.startsWith(".") && name.toLowerCase().endsWith(".txt") );
11.     }
12.   }
13.   );
14. }

Nice one Quark. I made this out of it :

1. for (File fileName : new File(sketchPath).listFiles(new FilenameFilter() {
2.     public boolean accept(File directory, String fileName) {
3.         return fileName.toLowerCase().endsWith(".txt");
4.     }})) {
5.     try {
6.     println(fileName.getCanonicalPath());
7.   }
8.   catch (Exception e) {
9.     println(e);
10.   }
11. }

Hey thanks a ton guys.  Im trying to make a quete and I needed a way of detecting all .txt files in a folder.