I managed to figure this out. All I had to do was to import the following packages:
import com.google.api.client.googleapis.auth.oauth2.GoogleCredential;
import com.google.api.services.plus.Plus;
import com.google.api.services.plus.PlusScopes;
And replace the following code:
GoogleAccessProtectedResource accessProtectedResource = new GoogleAccessProtectedResource(
response.accessToken, httpTransport, jsonFactory, clientId, clientSecret,
response.refreshToken);
Calendar service = new Calendar(httpTransport, accessProtectedResource, jsonFactory);
service.setApplicationName("YOUR_APPLICATION_NAME");
With this code:
GoogleCredential credential;
credential = new GoogleCredential.Builder().setTransport(httpTransport)
.setJsonFactory(jsonFactory).setServiceAccountId("[[INSERT SERVICE ACCOUNT EMAIL HERE]]")
.setServiceAccountScopes(Collections.singleton(PlusScopes.PLUS_ME))
.setServiceAccountPrivateKeyFromP12File(new File("key.p12"))
.build();
Plus plus = new Plus.Builder(httpTransport, jsonFactory, credential)
.setApplicationName("YOUR_APPLICATION_NAME")
.build();