it works!

snip.java

OAuthServiceSync o = createOAuthService();
TokenResponse t = o.getToken(new DwollaTypedBytes(new Gson(),
        new TokenRequest("client_id",
                "client_secret", "authorization_code",
                "redirect", "code")));

System.out.println("token: " + t.access_token);
System.out.println("refresh token: " + t.refresh_token);

TokenResponse r = o.refreshToken(new DwollaTypedBytes(new Gson(),
        new TokenRequest("id",
                "secret", "refresh_token", t.refresh_token)));

System.out.println("refreshed result: " + r.access_token);