I need to get user objects in many places, which contain many fields. After login, I want to save/store these user objects. How can we implement this kind of scenario?
If your Object is complex I'd suggest to Serialize/XML/JSON it and save those contents to the SD card. You can find additional info on how to save to external storage here:
http://developer.android.com/guide/topics/data/data-storage.html#filesExternal