How to check if logged in user session is valid in Android Parse SDK?

I am currently trying to setup user login and registration in my app. The first thing I am doing though is having my MainActivity check to see if a user is logged in and if they are, check if the session is valid. I am not sure if I am getting the ParseSession correctly, and if I am, I am not sure what to do at this point. (The docs are not very clear at showing how to accomplish this). Here is my code:

public class MainActivity extends AppCompatActivity {

    ParseUser currentUser;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Parse.initialize(new Parse.Configuration.Builder(this)
            .applicationId(getResources().getString(R.string.parse_app_id))
            .clientKey(getResources().getString(R.string.parse_client_key))
            .server(getResources().getString(R.string.parse_server_url))
            .build()
        );

        if (isFirstLaunch()) {
            Toast.makeText(this, "This is the first launch", Toast.LENGTH_LONG).show();
        }

        if (!isUserLoggedIn()) {
            Toast.makeText(this, "User is not logged in", Toast.LENGTH_LONG).show();
        }
    }

    private boolean isFirstLaunch() {
        return false;
    }

    private boolean isUserLoggedIn() {
        currentUser = ParseUser.getCurrentUser();
        if (currentUser == null) {
            return false;
        } else {
            ParseSession.getCurrentSessionInBackground(new GetCallback<ParseSession>() {
                @Override
                public void done(ParseSession object, ParseException e) {
                    /*
                     * if session is valid
                     * return true
                     * if session is invalid
                     * run ParseUser.logOut()
                     * return false
                     */
                }
            });
        }
    }

}