How to automate OAuth2 implicit flow using rest assured

Question

I've used the below code:

 OAuthClient client = new OAuthClient(new URLConnectionClient());

        OAuthClientRequest request =
                OAuthClientRequest.tokenLocation("url")
                        .setGrantType(GrantType.IMPLICIT)
                        .setClientId("clientid")
                        .setRedirectURI("redirecturl")
                        .setScope("scope")
                        .buildQueryMessage();

        request.addHeader("Accept", "application/json");
        request.addHeader("Content-Type", "application/json");

        System.out.println(request);


        String token = client.accessToken(request, OAuth.HttpMethod.POST, OAuthJSONAccessTokenResponse.class).getAccessToken();

Below error is occurred when run the code:

Exception in thread "main" OAuthProblemException{error='unsupported_response_type', description='Invalid response! Response body is not application/json encoded', uri='null', state='null', scope='null', redirectUri='null', responseStatus=0, parameters={}}
    at org.apache.oltu.oauth2.common.exception.OAuthProblemException.error(OAuthProblemException.java:63)
    at org.apache.oltu.oauth2.client.response.OAuthJSONAccessTokenResponse.setBody(OAuthJSONAccessTokenResponse.java:76)
    at org.apache.oltu.oauth2.client.response.OAuthClientResponse.init(OAuthClientResponse.java:92)
    at org.apache.oltu.oauth2.client.response.OAuthAccessTokenResponse.init(OAuthAccessTokenResponse.java:65)
    at org.apache.oltu.oauth2.client.response.OAuthClientResponse.init(OAuthClientResponse.java:101)
    at org.apache.oltu.oauth2.client.response.OAuthAccessTokenResponse.init(OAuthAccessTokenResponse.java:60)
    at org.apache.oltu.oauth2.client.response.OAuthClientResponse.init(OAuthClientResponse.java:120)
    at org.apache.oltu.oauth2.client.response.OAuthClientResponseFactory.createCustomResponse(OAuthClientResponseFactory.java:82)
    at org.apache.oltu.oauth2.client.URLConnectionClient.execute(URLConnectionClient.java:111)
    at org.apache.oltu.oauth2.client.OAuthClient.accessToken(OAuthClient.java:65)
    at TestCLass.main(TestCLass.java:57)

Process finished with exit code 1