@Override
  @NotNull
  public Collection<File> getFiles() {
    Collection<File> result = new ArrayList<>();
    for (int i = 0;  i < getChildCount(); i++){
      FileOrDirectoryTreeNode child = (FileOrDirectoryTreeNode)getChildAt(i);
      result.addAll(child.getFiles());
    }
    return result;
  }
@Override
  @NotNull
  public Collection<File> getFiles()