Android - Unable to save the login in my android app

Hi,

I am able to log the user in and retrieve the access token the first time and store it using SharedPreferences. However,  I want the users to log in automatically the next time they open my app (retrieve the access token silently).

I can always retrieve the access token stored in the app's local storage (SharedPreferences) but it might not be valid after a few hours/days. Is there a way to check if the access token in the local storage is valid and if not, retrieve a new access token without the user entering their dropbox credentials again.

Below is my code for retrieving the access token the first time:

private void dropboxLogin() {
        System.out.println("First Login");
        Auth.startOAuth2Authentication(MainActivity.this, APP_KEY);
}

    @Override
    protected void onResume() {
        super.onResume();
        SharedPreferences prefs = getSharedPreferences("dropbox-sample", MODE_PRIVATE);
        String accessToken = prefs.getString("access-token", null);
        if (accessToken == null) {
            accessToken = Auth.getOAuth2Token();
            if (accessToken != null) {
                prefs.edit().putString("access-token", accessToken).apply();
                initAndLoadData(accessToken);
            }
        } else {
            initAndLoadData(accessToken);
        }
        System.out.println("Access Token: " + accessToken);
        String uid = Auth.getUid();
        String storedUid = prefs.getString("user-id", null);
        if (uid != null && !uid.equals(storedUid)) {
            prefs.edit().putString("user-id", uid).apply();
        }
    }

Note that I am using Dropbox API v2.

Thanks in advance!