Question

I have an abstract class UserdataUpdater which extends Updater

Updater has a method declaration

 public abstract void processRow (Cluster cluster, IAppendOnlyData row);

Is there anyway to modify this method declaration inside UserdataUpdater to make it more specific, like

 public abstract void processRow (Cluster cluster, IUserData row);

IUserData extends IAppendOnlyData, because I want classes that extends UserdataUpdater to only take IUserData

Answer 1

No, you can’t. This would break the contract of the superclass, which says: this method accepts a IAppendOnlyData as second argument.

Remember that an instance of a subclass is also an instance of its superclass. So anyone could refer to the subclass instance as its superclass, and call the base method, passing a IAppendOnlyData, without knowing that the instance is actually a subclass instance.

Read more about the Liskov substitution principle.

The only way to do that is to make the superclass generic:

public class Updater<T extends IAppendOnlyData> {
    ...
    public abstract void processRow(Cluster cluster, T row);

}

public class UserdataUpdater extends Updater<IUserData> {
    @Override
    public void processRow(Cluster cluster, IUserData row) {
        ...
    }
}