Can't cast ArrayList class to ArrayList of interface

Question

Code structure

public interface ListItem
public abstract class Device implements ListItem
public class DeviceList extends ArrayList<Device>
public class ListAdapter extends BaseAdapter{
...
    public ArrayList<ListItem> items;
...
    }

I want to pass DeviceList to ListAdapter, but can't cast DeviceList to ArrayList< ListItem>. Why is that?

If Device implements ListItem, and DeviceList is an array list of Device, shouldn't I be able to cast it?

I try it like this:

ArrayList<ListItem> list2 = (ArrayList<ListItem>) device_list;

devicelist is of type Device and arraylist is of type listitem — Peter Haddad, Dec 11 '17 at 14:20
Yes, but Device implements ListItem, so I should be able to cast it? — Invader Zim, Dec 11 '17 at 14:26

score 4 · Accepted Answer · answered Dec 11 '17 at 14:27

4

That's a common misunderstanding. Although Device is a ListItem, a List of Devices is not a List of ListItems. Btw, this already had been answered, see for example How to cast List<Object> to List<MyClass> and How do you cast a List of supertypes to a List of subtypes?

answered Dec 11 '17 at 14:27

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Thanks, will accept answer as soon as I can [: – Invader Zim Dec 11 '17 at 14:29

Can't cast ArrayList class to ArrayList of interface

1 Answers1