import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.util.Formatter;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
​
public final class SignatureCalculator {
​
    private static final String HMAC_ALGORITHM = "HmacSHA256";
    private static final String CHARSET = "UTF-8";
​
    public static String calculateSignature(String x_Login, String x_Date, String secretKey, String body)
        throws IOException, InvalidKeyException, NoSuchAlgorithmException {
​
        // Create byte array with the required data for the signature.
        ByteArrayOutputStream bout = new ByteArrayOutputStream();
        bout.write(x_Login.getBytes(CHARSET));
        bout.write(x_Date.getBytes(CHARSET));
        bout.write(body.getBytes(CHARSET));
​
        // Calculate the signature.
        SecretKeySpec signingKey = new SecretKeySpec(secretKey.getBytes(), HMAC_ALGORITHM);
        Mac mac = Mac.getInstance(HMAC_ALGORITHM);
        mac.init(signingKey);
        byte[] signature = mac.doFinal(bout.toByteArray());
​
        // Create a String with the signature value.
        Formatter formatter = new Formatter();
        for (byte b : signature) {
            formatter.format("%02x", b);
        }
        return formatter.toString();
    }
}