code: public synchronized Mac getHmac256() throws InvalidKeyException {
          if (this.hmacSha256 == null) {
              // Initializes the HMAC-SHA256 Mac and SecretKey.
              try {
                  this.hmacSha256 = Mac.getInstance("HmacSHA256");
              }
              catch (final NoSuchAlgorithmException e) {
                  throw new IllegalArgumentException();
              }
              this.hmacSha256.init(new SecretKeySpec(this.key, "HmacSHA256"));
          }
          return this.hmacSha256;
      }
docs: Gets the HmacSha256 associated with the account key.
  
  @return A <code>MAC</code> created with the account key.
  
  @throws InvalidKeyException
  If the key is not a valid storage key.