code: public static boolean writeString(String filePath, String content, boolean append) throws IOException {
          return writeString(filePath != null ? new File(filePath) : null, content, append);
      }
docs: write file
  
  @param filePath
  @param content
  @param append   is append, if true, write to the end of file, else clear content of file and write into it
  @return return true
  @throws IOException if an error occurs while operator FileWriter