GrantType Parse Exception

I try to implement OAuth2 TOKEN Endpoint with java servlet, the sample code as follows

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        try {
            final HTTPRequest httprequest = new HTTPRequest(request);
            final TokenRequest tokenrequest = TokenRequest.parse(httprequest);
            AuthorizationGrant ag = tokenrequest.getAuthorizationGrant();
            System.out.println("Grant Type : " + ag.getType().getValue());
            final ClientSecretBasic clientsecretbasic = (ClientSecretBasic)tokenrequest.getClientAuthentication();
            System.out.println("Client ID : " + clientsecretbasic.getClientID() + " Client Secret : " + clientsecretbasic.getClientSecret().getValue());

        } catch (ParseException | IOException ex) {
            System.out.println(ex);
        }
    }

When I using Google OAuth2 Playground to fetch the token , it failed and show error message below:

com.nimbusds.oauth2.sdk.ParseException: Unsupported grant type: authorization_code

But when I look back to the source code, I could not figure what's wrong. The attachments it the log.

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