code: public void checkRequiredProperties(String... propNames) throws ProxyAuthException {
          StringBuilder sb = new StringBuilder();
          for (String propertyName : propNames) {
              if (properties.get(propertyName) == null) {
                  sb.append(propertyName).append(' ');
              }
          }
          if (sb.length() > 0) {
              sb.append("property(ies) missing in request");
              throw new ProxyAuthException(sb.toString());
          }
      }
docs: Check if the given property(ies) is(are) set. Otherwise throws a
  {@link ProxyAuthException}.