Why return null length in Android mode?

This function in java mode works well.. but in android mode returns null length of folder.. any solution??

PImage[] loadImages(String nameFolder) {
  java.io.File folder = new java.io.File(dataPath("")+"/"+nameFolder);
  String[] filenames = folder.list();
  //println(filenames.length);
  //println(dataPath("")+"/"+nameFolder);
  PImage[] img = new PImage[filenames.length];
  for (int i = 0; i < filenames.length; i++) {
    //println("Loading: "+filenames[i]);
    img[i] = loadImage(dataPath("") + "/" + nameFolder + "/" + filenames[i]);
  }
  return img;
}