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