Java – generics using arrayadapters

I encountered a problem trying to create a generic arrayadapter This may be related to my limited understanding of generics in Java, and I hope someone can let me be straightforward Basically, I have an abstract basic adapter class:

public abstract class BaseAdapter<T> extends ArrayAdapter<T>
{
    ....
    private List<T> items;
    ....
    protected abstract List<T> build(JSONArray jsonArray);
    ....
    @Override
    public T getItem(int position)
    {
        return items.get(position);
    }
    ....
}

Then I have an adapter that extends this base class:

public class MyAdapter extends BaseAdapter<BaseModel>
{
    ....
    @Override
    protected List<BaseModel> build(JSONArray jsonArray)
    {
        if (useBaseModel())
        {
            return BaseModel.buildBaseModels(jsonArray); //returns List<BaseModel>
        }
        else
        {
            return SubclassModel.buildSubclassModels(jsonArray); //returns List<SubclassModel>
        }
    }
    ....
}

The model is defined as:

public class BaseModel{...}

public class SubclassModel extends BaseModel{....}

This does not compile because even if subclassmodel extends basemodel, their list is different This post (most effective way to cast list < subclass > to list < baseClass >) explains this and says to use it instead:

List<? extends BaseModel>

When I change to:

public class MyAdapter extends BaseAdapter<? extends BaseModel> 
{
    ...
    protected List<? extends BaseModel> build(JSONArray jsonArray)
    ...
 }

The error on the build method disappeared, but now there is a class level error description: getitem (int) and Android. Net in baseadapter widget. Getitem (int) conflict in arrayadapter; An incompatible return type was attempted

It seems that getitem should return a type? Extend basemodel, who is it – can anyone provide advice?

thank you!

Solution

Well, my java SDK doesn't even let me compile

public class MyAdapter extends BaseAdapter<? extends BaseModel>

It expects an extended unbounded class or interface But it doesn't matter. If this is an option, the solution to the problem is to change the baseadapter class:

public abstract class BaseAdapter<T> extends ArrayAdapter<T>
{
    ....
    private List<T> items;
    ....
    protected abstract List<? extends T> build(JSONArray jsonArray);
    ....
    @Override
    public T getItem(int position)
    {
        return items.get(position);
    }
}

then

public class MyAdapter extends BaseAdapter<BaseModel> 
{
    ...
    protected List<? extends BaseModel> build(JSONArray jsonArray)
    ...
}

Work with previous code

Edit:

Forget to explain why your first solution doesn't work:

Language spec says

This means that although subclassmodel extends basemodel, it does not mean that list extends list