Why do we use single adapter per class in android?

Question

I have just started learning android using Big Nerd Ranch Guide. In a project, we were to implement a listView and get the data from an array using an ArrayAdapter, as the crime objects were stored in some list.

protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
getActivity().setTitle(R.string.crimes_title);
mCrimes = CrimeLab.get(getActivity()).getCrimes();
ArrayAdapter<Crime> adapter =
new ArrayAdapter<Crime>(getActivity(),
android.R.layout.simple_list_item_1,
mCrimes);
setListAdapter(adapter);
}

After that, we were to create custom list to display the title as well as the checkbox and date so we created a custom layout. Now, we used a different adapter for that purpose.

private class CrimeAdapter extends ArrayAdapter<Crime> {
public CrimeAdapter(ArrayList<Crime> crimes) {
super(getActivity(), 0, crimes);
}

My question is why cant we use the same adapter that we used earlier?I mean we can just give it new values or create a new object.I searched the internet and found that we can create multiple adapters with some restrictions. why we extend it to ArrayAdapter? cant we just do it like we did previously by creating the adapter? Please explain in detail, as I have just started.

Thanks.

Answer 1

Let us talk about the list view adapter implementation in android first of all in your first example you used the default implementation of listview adapter in android which provide you with list view of text

but most of the time this is not enough for your user interface you want to add an image a checkbox like in your second example so android OS gives you the ability to make your own custom cell (each row in list) that's why you would find yourself in a need to extend array adapter

in your first example you used

android.R.layout.simple_list_item_1

which is a layout containing only textview

and you provide a data source mCrimes

so when you extend array adapter you do the same you provide your own layout which represent the cell and a data source which will populate your list

hope that clarify it for you and welcome aboard :)