Java Generic: Why compiler can not resolve type T to inherited type in method?

Question

I've got classes

public class DiaryItem extends AbstractInfoElement { ... };

public class EventItem extends AbstractInfoElement { ... };

public class StickerItem extends AbstractInfoElement { ... };

public class TodoItem extends AbstractInfoElement { ... };

I've got overridden methods to do something with this parameters:

class DBEngine {

    public long add(EventItem item){ ... };

    public long add(StickerItem item){ ... };

    public long add(TodoItem item){ ... }

}

Then I realized template class to manipulate types in generic way

public class AddItemCommand<T extends AbstractInfoElement>{

    private final T item;

    public AddItemCommand(T item) {
        this.item = item;
    }

    void doSmth() {
      add(item);
    }
}

I suppose compiler resolve T as needed type, but I got issues:

Error:(78, 45) error: no suitable method found for add(T)
method DBEngine.add(DiaryItem) is not applicable
(argument mismatch; T cannot be converted to DiaryItem)
method DBEngine.add(EventItem) is not applicable
(argument mismatch; T cannot be converted to EventItem)
method DBEngine.add(StickerItem) is not applicable
(argument mismatch; T cannot be converted to StickerItem)
method DBEngine.add(TodoItem) is not applicable
(argument mismatch; T cannot be converted to TodoItem)
where T is a type-variable:
T extends AbstractInfoElement declared in class AddItemCommand

My goal was to avoid overcoding in those cases:

AddItemCommand<DiaryItem> cmdDiary = new AddItemCommand<DiaryItem>(new DiaryItem);
cmdDiary.doSmth();

should call

DBEngine.add(DiaryItem item);

and

AddItemCommand<TodoItem> cmdTodo = new AddItemCommand<TodoItem>(new TodoItem);
cmdTodo.doSmth();

should call

DBEngine.add(TodoItem item);

That's all, but doesn't work ... Those issues are at compile time...

BTW... Excuse my poor english

Answer 1

From your error messages it appears that you only have add methods for the concrete subclasses, DiaryItem , EventItem , StickerItem , and TodoItem , but then you attempt to call add with an argument of type T extends AbstractInfoElement .

That does not work as T ("any subclass of AbstractInfoElement") does not match either of the specific subclasses.

Edit: note that Java only does dispatch on static type (ie, which of the overloaded methods to call is selected based on the type known at compile-time), so it has no way of knowing if T matches one of the subclasses.

(see eg, How does Java method dispatch work with Generics and abstract classes? , Java Generic / Type Dispatch Question , Method overloading and generics )