import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.io.UnsupportedEncodingException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.util.Base64;
import java.util.TreeMap;
public class VadsSignatureExample {
 /**
  * Build signature (HMAC SHA-256 version) from provided parameters and secret key.
  * Parameters are provided as a TreeMap (with sorted keys).
  */
 public static String buildSignature(TreeMap<String, String> formParameters, String
  secretKey) throws NoSuchAlgorithmException, InvalidKeyException, UnsupportedEncodingException {
   // Build message from parameters
   String message = String.join("+", formParameters.values());
   message += "+" + secretKey;
   // Sign
   return hmacSha256Base64(message, secretKey);
  }	                    
    /**
    * Actual signing operation.
    */
 public static String hmacSha256Base64(String message, String secretKey) throws
  NoSuchAlgorithmException, InvalidKeyException, UnsupportedEncodingException {
   // Prepare hmac sha256 cipher algorithm with provided secretKey
   Mac hmacSha256;
   try {
    hmacSha256 = Mac.getInstance("HmacSHA256");
   } catch (NoSuchAlgorithmException nsae) {
    hmacSha256 = Mac.getInstance("HMAC-SHA-256");
   }
   SecretKeySpec secretKeySpec = new SecretKeySpec(secretKey.getBytes("UTF-8"), "HmacSHA256");
   hmacSha256.init(secretKeySpec);
   // Build and return signature
   return Base64.getEncoder().encodeToString(hmacSha256.doFinal(message.getBytes("UTF-8")));
  }
}