Hash

String stringToHash = amount=100&callbackUrl=https://checkout.seerbit.com/?m=EQREZEhyRn&country=NG&currency=NGN&email=kolawole@gmail.com&paymentReference=5fgj77treo&productDescription=RiceandBeans&productId=Foods&publicKey=EQREZEhyR

String hashString = stringToHash + privateKey;

Hash = Sha256(hashString);

public static String Sha256(String rawKey) throws ProcessingException {
    try {
      MessageDigest md = MessageDigest.getInstance("SHA-256");
      byte[] encrypted = md.digest(rawKey.getBytes());
      return new String(Hex.encodeHex(encrypted));
    } catch (NoSuchAlgorithmException ex) {
      String errorMessage = "Unable to hash this string";
      throw new ProcessingException(errorMessage);
    }
  }