Returns an array with all filenames found in the given path filtered by the given extension (optional).

processing-list-files-in-folder.pde

import java.io.File;
import java.io.FilenameFilter;

// returns all filenames
String[] processFolder(String path) {
  return processFolder(path, null);
}
// returns all filenames filtered by extension
String[] processFolder(String path, final String ext) {
  java.io.File folder = new java.io.File(path);

  if (ext == null) {
    return folder.list();
  }
  
  java.io.FilenameFilter extFilter = new java.io.FilenameFilter() {
    public boolean accept(java.io.File dir, String name) {
      return name.toLowerCase().endsWith("." + ext);
    }
  };

  return folder.list(extFilter);
}