Get Invoice Details

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.HashMap;

public class GetInvoiceDetails {
    /** */
    private static final String privateKey = "OPAYPRV16357783346230.07653446106051542";
    /** */
    private static final String endpoint = "https://sandboxapi.opaycheckout.com";
    /** */
    private static final String merchantId = "256612345678901";

    public static void main(String[] args) throws Exception {
        String addr = endpoint + "/api/v1/international/invoice/getInvoiceDetails";
        Gson gson = new Gson();
        HashMap order = new HashMap<>();
        order.put("invoiceNo","20839397");

        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);
    }
}