Subclass cannot be applied to Super class Java

Question

I have the following abstract class:

public abstract class MyObject<T extends DescriptionItem> {

    protected abstract MyObject generate(T item);
}

which has the following subclass:

public class AudioObject extends MyObject<PlayNowDescriptionItem> {

    @Override
    protected MyObject generate(PlayNowDescriptionItem item) {
        //do something
    }
}

PlayNowDescriptionItem extends DescriptionItem. I also have a factory class for generating objects.

public class ObjectFactory {

 public MyObject generateCastObject(DescriptionItem item) {
    if (item instanceof PlayNowDescriptionItem) {
        return new AudioObject(context).generate(item);
}

I thought that this would work fine because PlayNowDescriptionItem is a child of DescriptionItem, but i get an error on the following line.

     return new AudioObject(context).generate(item);

PlayNowDecriptionItem in AudioObject cannot be applied to DescriptionItem.

Can anyone see what i'm doing wrong here?

Answer 1

You need to cast your DescriptionItem to PlayNowDescriptionItem like this:

return new AudioObject(context).generate((PlayNowDescriptionItem)item);

as you defined the generate method to accept a subclass of DescriptionItem (please note that an object of class PlayNowDescriptionItem is also an object of class DescriptionItem but the contrary is not always valid, eg. could be a subclass PlayAfterDescriptionItem that's is not a PlayNowDescriptionItem )