private List<Pair<String, String>> getParameters(PayUDetails details) {
   List<Pair<String, String>> parameters = new ArrayList<Pair<String, String>>();
   parameters.add(new Pair<String, String>("MERCHANT", details.getMerchant()));
   parameters.add(new Pair<String, String>("ORDER_REF", details.getReference()));
   parameters.add(new Pair<String, String>("ORDER_AMOUNT", details.getAmount()));
   parameters.add(new Pair<String, String>("ORDER_CURRENCY", details.getCurrency()));
   parameters.add(new Pair<String, String>("IRN_DATE", details.getDate()));
   parameters.add(new Pair<String, String>("ORDER_HASH", getHash(details)));
   parameters.add(new Pair<String, String>("REF_URL", details.getReferenceUrl()));
   return parameters;
 }
 private String getHash(PayUDetails details) {
   StringBuilder hashString = new StringBuilder();
   hashString.append(details.getMerchant().length());
   hashString.append(details.getMerchant());
   hashString.append(details.getReference().length());
   hashString.append(details.getReference());
   hashString.append(details.getAmount().length());
   hashString.append(details.getAmount());
   hashString.append(details.getCurrency().length());
   hashString.append(details.getCurrency());
   hashString.append(details.getDate().length());
   hashString.append(details.getDate());
   return hmacMD5(hashString.toString(), secretKey);
 }