docs: public static boolean delete(File d) {
          if (null == d || !d.exists())
              return true; // by definition
  
          if (d.isDirectory()) {
              File[] files = d.listFiles(); // and directories
              if (null != files) {
                  for (File f : files) {
                      if (f.isDirectory()) {
                          delete(f);
                      } else {
                          f.delete();
                      }
                  }
              }
          }
          return d.delete();
      }