Why do I not need to use Adapter.notifyDataSetChanged()?

Question

the contactsList is empty until the readContacts()method was executed, in other words, when contactsView.setAdapter(adapter) was executed, the contactsList is empty, so why this code still can show contacts' info correctly?

public class MainActivity extends AppCompatActivity {

ListView contactsView;
ArrayAdapter<String> adapter;
List<String> contactsList = new ArrayList<String>();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    contactsView = (ListView) findViewById(R.id.contacts_list);
    adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, contactsList);
    contactsView.setAdapter(adapter);
    readContacts();
}

private void readContacts() {
    Cursor cursor = null;
    try {
        cursor = getContentResolver().query(
                ContactsContract.CommonDataKinds.Phone.CONTENT_URI,
                null, null, null, null);
        while (cursor.moveToNext()) {
            String displayName = cursor.getString(cursor.getColumnIndex(
                    ContactsContract.CommonDataKinds.Phone.DISPLAY_NAME
            ));
            String number = cursor.getString(cursor.getColumnIndex(
                    ContactsContract.CommonDataKinds.Phone.NUMBER
            ));
            contactsList.add(displayName + "\n" + number);
        }
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        if (cursor != null) {
            cursor.close();
        }
    }
    }
}

but if i add something like this, i have to call notifyDataSetChanged():

    add.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            contactsList.add("blabla");
            adapter.notifyDataSetChanged();
        }
    });

add is button. now that the android would call the method automatically, why when remove the adapter.notifyDataSetChanged(); the UI couldn't refresh?

Answer 1

That method is called internally in the Android framework. You do not have to call it explicitly.

source: https://www.udacity.com/course/developing-android-apps--ud853