code: protected void checkHttpSchemeSpecificPartSlashPrefix(final URI base,
              final String scheme, final String schemeSpecificPart)
      throws URIException {
          if (scheme == null || scheme.length() <= 0) {
              return;
          }
          if (!scheme.equals("http") && !scheme.equals("https")) {
              return;
          }
          if ( schemeSpecificPart == null 
                  || !schemeSpecificPart.startsWith("//")) {
              // only acceptable if schemes match
              if (base == null || !scheme.equals(base.getScheme())) {
                  throw new URIException(
                          "relative URI with scheme only allowed for " +
                          "scheme matching base");
              } 
              return; 
          }
          if (schemeSpecificPart.length() <= 2) {
              throw new URIException("http scheme specific part is " +
                  "too short: " + schemeSpecificPart);
          }
      }
docs: If http(s) scheme, check scheme specific part begins '//'.
  @throws URIException
  @see http://www.faqs.org/rfcs/rfc1738.html Section 3.1. Common Internet
  Scheme Syntax