i am using a FirebaseecyclerOption, but i don't know the use of it and also tell me how it perform it to populate and store data in model class, like it execute each time or its store all the data in the option and populate it into the model class please explain thanks, and another question is how we can get only limited items from the database.
The FirebaseRecyclerOptions class controls how FirebaseUI populates the RecyclerView with data from the Realtime Database. At a minimum you pass it a query or location to get the data from, and a Java class that will be instantiated to hold the data from each row in the view. For more on this, see the documentation on using the FirebaseRecyclerAdapter.
To limit the data shown you have two options. You can either point to a location with less data, or use a query to limit what data is retrieved. The FirebaseUI documentation on querying shows an example of that, and you find even more examples in the regular Firebase documentation on sorting and filtering data.