code: public Future<AuthenticationResult> acquireTokenByRefreshToken(
              final String refreshToken, final String clientId,
              final ClientAssertion clientAssertion,
              final AuthenticationCallback callback) {
          return acquireTokenByRefreshToken(refreshToken, clientId, clientAssertion,
                  null, callback);
      }
docs: Acquires a security token from the authority using a Refresh Token
  previously received.
  
  @param refreshToken
  Refresh Token to use in the refresh flow.
  @param clientId
  Name or ID of the client requesting the token.
  @param clientAssertion
  The client assertion to use for client authentication.
  @param callback
  optional callback object for non-blocking execution.
  @return A {@link Future} object representing the
  {@link AuthenticationResult} of the call. It contains Access
  Token, Refresh Token and the Access Token's expiration time.