I constantly met errors of "The output stream to the file must be closed." However, as shown in my current solution, I am pretty sure I have explicitly closed all file output stream, writer , or reader objects. In addition, referring to https://codegym.cc/help/4001 where we shared a similar solution, RomanLevel 41 also suggested to not use recursion to visit all file tree, which is what I did - use queue to iterate everything. But still got same output stream close error. Am I missing anything ? Thanks!