Payment Gateway integration in JSP

public static String calculateChecksum(String secretKey, String allParamValue) throws Exception { 
	byte[] dataToEncryptByte = allParamValue.getBytes();
    byte[] keyBytes = secretKey.getBytes();
    SecretKeySpec secretKeySpec = new SecretKeySpec(keyBytes, "HmacSHA256");
    Mac mac = Mac.getInstance("HmacSHA256");
    mac.init(secretKeySpec);
    byte[] checksumByte = mac.doFinal(dataToEncryptByte);
    String checksum = toHex(checksumByte);
    
    return checksum;
  }

private static String toHex(byte[] bytes)
  {
    StringBuilder buffer = new StringBuilder(bytes.length * 2);
    
    byte[] arrayOfByte = bytes;
    int j = bytes.length; 
    for (int i = 0; i < j; i++) { 
      Byte b = Byte.valueOf(arrayOfByte[i]);
      String str = Integer.toHexString(b.byteValue());
      int len = str.length();
      if (len == 8)
      {
        buffer.append(str.substring(6));
      } else if (str.length() == 2) {
        buffer.append(str);
      } else {
        buffer.append("0" + str);
      }
    }
    return buffer.toString();
  }

public static String getAllNotEmptyParamValue(HttpServletRequest request)
  {
    Enumeration<String> en = request.getParameterNames();
    List<String> list = Collections.list(en);
    List<String> checksumsequence= new ArrayList<>(Arrays.asList("amount","bankid","buyerAddress",
            "buyerCity","buyerCountry","buyerEmail","buyerFirstName","buyerLastName","buyerPhoneNumber","buyerPincode","buyerState","currency","debitorcredit","merchantIdentifier","merchantIpAddress","mode","orderId","product1Description","product2Description","product3Description","product4Description","productDescription","productInfo","purpose","returnUrl","shipToAddress","shipToCity","shipToCountry","shipToFirstname","shipToLastname","shipToPhoneNumber","shipToPincode","shipToState","showMobile","txnDate","txnType","zpPayOption"));
    List<String> Parameterinseq=new ArrayList<>();
    for(String s:checksumsequence)
    {
        if(list.contains(s))
        {	
          Parameterinseq.add(s);
        }
    }

    
    String allNonEmptyParamValue = "";
    for (int i = 0; i < Parameterinseq.size(); i++) {
      String paramName = (String)Parameterinseq.get(i);
      String paramValue = "";
      //if(request.getParameter(paramName)==""){
      
        paramValue = request.getParameter(paramName);
      if (!paramValue.equals(""))
      {
        allNonEmptyParamValue = allNonEmptyParamValue+paramName+"="+ paramValue + "&";
      }
    }
    System.out.println(allNonEmptyParamValue);
    return allNonEmptyParamValue;
  }

public static boolean verifyChecksum(String secretKey, String allParamVauleExceptChecksum, String checksumReceived) throws Exception
  {
    byte[] dataToEncryptByte = allParamVauleExceptChecksum.getBytes();
    byte[] keyBytes = secretKey.getBytes();
    SecretKeySpec secretKeySpec = new SecretKeySpec(keyBytes, "HmacSHA256");
    Mac mac = Mac.getInstance("HmacSHA256");
    mac.init(secretKeySpec);
    byte[] checksumCalculatedByte = mac.doFinal(dataToEncryptByte);
    String checksumCalculated = toHex(checksumCalculatedByte);
    if (checksumReceived.equals(checksumCalculated)) {
      return true;
    }
    return false;
  }