Question

Ok so I know that you can’t have an abstract static method, although I see this as a limitation personally. I also know that overriding static methods is useless because when I am dealing with say MyList<T extends ObjectWithId> and my object has an abstract class with a static method that gets overridden in it’s subclasses, T doesn’t exist at runtime so ObjectWithId‘s static method would be called instead of the subclass.

So here is what I have:

class PersistentList<T extends ObjectWithId> implements List<T>{

}

where ObjectWithId is:

abstract ObjectWithId{
    public abstract long getId();
}

Now the issue is that my PersistentList is meant to be stored on hard disk, hence the name, and in reality will only store ids of objects it holds. Now when I want to implement the

@Override
public T get(int index) {

}

method of PersistentList, what I want is for my program to use the id it has stored for index and call a static method objectForId(long id) which would be implemented in each subclass of ObjectWithId. It can’t be a instance method because there is no instance yet, the point is to load the instance from the hard disk using the id. So how should it be implemented? One option is to have ObjectWithId have a constructor ObjectWithId(long id) implemented in each subclass, but T doesn’t exist at runtime so how would I instantiate it? I know I could pass Class<T> object in the constructor of PersistentList but I would prefer if the constructor did not have any arguments, but I don’t think there is a way to get the class of T without explicitly passing it in right?

I hope this is a better explanation, sorry for the ambiguous question I started with.

Answer 1

While passing the Class<T> as a constructor argument, it does not really solves your problem. You then have access to the class, but to get access to the static method defined on the class you will have to use generics (unless somebody else knows a way to call a static method defined on a class from a Class object).

I would define a new generic interface which contains a generic method objectForID, something like

public interface ObjectRetriever<T>{
  public T objectForID( long aID );
}

and adjust the constructor of the PersistentList to take such a ObjectRetriever instance as parameter. This ObjectRetriever can then be used to restore the objects based on their ID.