code: protected int compareFiles(@NonNull File lhs, @NonNull File rhs) {
          if (lhs.isDirectory() && !rhs.isDirectory()) {
              return -1;
          } else if (rhs.isDirectory() && !lhs.isDirectory()) {
              return 1;
          } else {
              return lhs.getName().compareToIgnoreCase(rhs.getName());
          }
      }
docs: Compare two files to determine their relative sort order. This follows the usual
  comparison interface. Override to determine your own custom sort order.
  <p/>
  Default behaviour is to place directories before files, but sort them alphabetically
  otherwise.
  
  @param lhs File on the "left-hand side"
  @param rhs File on the "right-hand side"
  @return -1 if if lhs should be placed before rhs, 0 if they are equal,
  and 1 if rhs should be placed before lhs