Q:

The method onListItemClick(ListView, View, int, long) of type MainActivity must override or implement a supertype method

I just changed a ListActivity to Activity, and everything works fine except the code that makes the activity do something if click on List Item (onListItemClick).   
I get this error:
The method onListItemClick(ListView, View, int, long) of type MainActivity must override or implement a supertype method

@Override
protected void onListItemClick(ListView lv, View v, int position, long id) {
        super.onListItemClick(lv, v, position, id);

        // Do something

      }

( This class was extending ListActivity and everything works fine, but now I want it to extend Activity).
Everything works fine except for this code.

A:

Your issue is that you'd extends as ListActivity and after some time you had removed ListActivity and extend your class to Activity. onListItemClick is an override method of ListActivity which is not available in Activity 
Solution : If you want to use ListItemClick in Activity so you can do it by following ways else you need to extends your MainClass with ListActivity instead of Activity
There are 2 ways to setOnItemClickListener to the ListView while using Activity
- Implement OnItemClickListener to your Activity
public class MyActivity extends Activity implements OnItemClickListener {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public void onItemClick(AdapterView<?> arg0, View arg1, int arg2, long arg3) {

    }
}

- Write ListView's setOnItemClickListener
mListView.setOnItemClickListener(new OnItemClickListener() {

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
    }
});

