google oauth2 authentication error

Question

I am trying to build an android app to insert and retrieve events from google calendar using Google calendar API.

`GoogleAccountCredential mCredential;
mCredential = GoogleAccountCredential.usingOAuth2(
                getApplicationContext(), Arrays.asList(SCOPES))
                .setBackOff(new ExponentialBackOff());`

whenever I try to access the API it gives me the error UNREGISTERED_ON_API_CONSOLE. I have followed the steps on calendar API Page to register API key on developer console