code: public static BigInteger extractExpiryFromCltvPaymentChannel(Script script) {
          return Script.castToBigInteger(script.chunks.get(4).data, 5, false);
      }
docs: Retrieves the locktime from a LOCKTIMEVERIFY transaction. It's important that the script is in the correct form,
  so you will want to guard calls to this method with {@link #isSentToCltvPaymentChannel(Script)}.