I know a list view needs an adapter, but WHY what is the purpose of it, and can it show lists without an adapter ?
2 Answers
From the docs:
An
Adapter
object acts as a bridge between anAdapterView
and the underlying data for that view. TheAdapter
provides access to the data items. TheAdapter
is also responsible for making aView
for each item in the data set.
The ListView
needs to know which items to show, and needs to get View
s for these items. The Android team chose to implement this using Adapter
s.
Note that for showing simple lists of String
s, there is the ArrayAdapter class.

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An adapter manages the data model and adapts it to the individual rows in the list view.
Filtering and sorting of the data is handled by the adapter
.
The notifyDataSetChanged()
method on the adapter is called if the data has changed or if new data is available.
The notifyDataSetInvalidated()
method is called if the data is not available anymore.