I have a list defined like this:
List<MyBean> beanList = getList();
getList() returns type List<MyBean>.
Now, I want to pass the result to a method that receives List<Idisplayable>
and MyBean does implements Idisplayable
This causes a compiler error.
Now it would be stupid to iterate over beanList just to cast it into Idisplayable. suggestions?
if the method that takes a List<IDisplayable> doesn't want to add anything to the list, then it should accept a List<? extends IDisplayable> instead, exactly to allow what you're trying to do.
The basic problem is that a List<MyBean> is not the same thing (or even assignment-compatible with) a List<IDisplayable>.
A List<IDisplayable> promises two things: It will only ever return IDisplayable objects and it will accept (via add()) all IDisplayable objects.
A List<MyBean> also only ever returns IDisplayable, but it will not accept any IDisplayable that is not also a MyBean (say, a AnotherDisplayable, for example).
So if a method only iterates over the content of a list (effectively being read-only) or only does a limited set of manipulation (removing objects is fine, as is adding null), then it should accept List<? extends IDisplayable> instead. That syntax means "I accept any List that will only ever return IDisplayable objects, but I don't care about the specific type".
null. Good point
Make your method accept <? extends Idisplayable>
public void myMethod(List<? extends Idisplayable> list);
You have 2 options:
If you have access to the method and it does not change the list, only in this case you may update the signature:
public void myMethod(List<? extends Idisplayable> list);
As an alternative option you may try:
List<Idisplayable> list = new ArrayList<Idisplayable>();
list.addAll(getList());
and then pass list to your method that takes List<Idisplayable> list
The point is that Collection.addAll is declared as:
addAll(Collection<? extends E> c), as a result, you may pass here both List<Idisplayable> and List<MyBean>
