code: public static String paramsToString(String charset, Map<String, String> params) {
          StringBuffer sb = new StringBuffer();
          try {
              for (Map.Entry<String, String> item : params.entrySet()) {
                  sb.append("&");
                  sb.append(URLEncoder.encode(item.getKey(), charset));
                  sb.append("=");
                  sb.append(URLEncoder.encode(item.getValue(), charset));
              }
          } catch (UnsupportedEncodingException e) {
              e.printStackTrace();
          }
          return sb.substring(1);
      }
docs: convert parameters to string
  
  @param charset
  编码
  @param params
  parameter
  @return 拼接后的字符串