getting data from a database vs getting data from a hash map

This depends on the number of products. A HashMap resides in RAM, a database resides on the disk.

This also depends on the number of queries per second and the nature of the queries. Database developers have put a lot of effort to support indexing and filtering; if you need that, reuse is better than re-inventing.

Whatever approach you choose, you must remember that an Android application process may be killed at any moment (e.g. when memory is needed for another process), and your code is guaranteed only to receive onPause() (and not onDestroy() or onStop(), see the activity life cycle). If necessary, the application will be restarted later, but all data kept in RAM (and not saved) will be of course lost. Therefore, in either onPause() or in onSaveInstanceState() (what bundles are for) you must save the application state. In your case this may mean having to save the HashMap and all auxiliary data structures.

OTOH, if the number of products is small (and is expected to remain small), a database can be an overkill; if you just need to choose one of 10 items, writing a loop is faster than doing all the database support.

One more important note: an Activity is a Controller from the MVC viewpoint (and as to the View, you usually create and XML and reuse the existing framework classes, although you could program custom controls). An Activity is re-created each time the device orientation changes and in some other cases. But your data belong to the Model part. They must survive the orientation change. So they must not be owned by an Activity.

To answer

a. HashMap will no doubt win over sqlite in terms of memory operations, hence it will run faster. But it will not provide you consistency, Mobile being a volatile environment, OS is bound to kill an application in background, or in foreground if its memory requirement are not met, in such scenarios you might loose all the important updates made before committing it to a permanent storage.

b. SQLITE is slow compared to Map, but is reliable it will make sure all the commits to your data is saved properly, even though app crashes db will guarantee to restore data you committed before that, Map certainly lacks this functionality.

c. Considering a Scenario when you have loaded data into a MAP to enable faster operation and performing sync with database whenever you record any delta to your data, this scenario is very plausible and can perform excellently if designed properly.

To conclude, You should proceed with DB + MAp operation, this will make your app running smooth if there is lots of database operation involved, just need to make sure keep data + app seperate to eleborate dont make your app dependant on loading of data initially.