code: public String join(String separator) throws JSONException {
  		int len = this.rawList.size();
  		StringBuilder sb = new StringBuilder();
  
  		for (int i = 0; i < len; i += 1) {
  			if (i > 0) {
  				sb.append(separator);
  			}
  			sb.append(InternalJSONUtil.valueToString(this.rawList.get(i)));
  		}
  		return sb.toString();
  	}
docs: JSONArray转为以<code>separator</code>为分界符的字符串
  
  @param separator 分界符
  @return a string.
  @throws JSONException If the array contains an invalid number.