Bank USSD Payment

import com.google.gson.Gson;
import org.apache.commons.codec.binary.Hex;

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.util.TreeMap;
import java.util.UUID;

public class BankUssd {

    private static final String privateKey = "OPAYPRV1641***********926";

    private static final String endpoint = "https://testapi.opaycheckout.com";

    private static final String merchantId = "256622011275597";

    public static void main(String[] args) throws Exception {
        String addr = endpoint + "/api/v1/international/payment/create";
        Gson gson = new Gson();
        TreeMap order = new TreeMap<>();
        TreeMap amount = new TreeMap<>();
        amount.put("currency","NGN");
        amount.put("total",400);
        order.put("amount",amount);
        order.put("bankCode", "033");
        order.put("callbackUrl","https://testapi.opaycheckout.com/api/v1/international/print");
        order.put("country","NG");
        order.put("customerName","customerName");
        order.put("payMethod","BankUssd");
        TreeMap product = new TreeMap<>();
        product.put("description","dd");
        product.put("name","name");
        order.put("product",product);
        order.put("reference", UUID.randomUUID().toString());
        order.put("userPhone","+1234567879");

        String requestBody = gson.toJson(order);
        System.out.println("--request:");
        System.out.println(requestBody);
        String oPaySignature = hmacSHA512(requestBody, privateKey);
        System.out.println("--signature:");
        System.out.println(oPaySignature);

        URL url = new URL(addr);
        HttpURLConnection con = (HttpURLConnection)url.openConnection();
        con.setRequestMethod("POST");
        con.setRequestProperty("Content-Type", "application/json; utf-8");
        con.setRequestProperty("Authorization", "Bearer "+oPaySignature);
        con.setRequestProperty("MerchantId", merchantId);
        con.setDoOutput(true);
        OutputStream os = con.getOutputStream();
        byte[] input = requestBody.getBytes(StandardCharsets.UTF_8);
        os.write(input, 0, input.length);
        BufferedReader br = new BufferedReader(new InputStreamReader(con.getInputStream(), StandardCharsets.UTF_8));
        StringBuilder response = new StringBuilder();
        String responseLine = null;
        while ((responseLine = br.readLine()) != null) {
            response.append(responseLine.trim());
        }

        System.out.println("--response:");
        System.out.println(response.toString());
        //close your stream and connection
    }

    public static String hmacSHA512(final String data, final String secureKey) throws Exception{
        byte[] bytesKey = secureKey.getBytes();
        final SecretKeySpec secretKey = new SecretKeySpec(bytesKey, "HmacSHA512");
        Mac mac = Mac.getInstance("HmacSHA512");
        mac.init(secretKey);
        final byte[] macData = mac.doFinal(data.getBytes());
        byte[] hex = new Hex().encode(macData);
        return new String(hex, StandardCharsets.UTF_8);
    }
}