Question

I’ve recently learned like 3 new languages and I’m starting to get them confused. I haven’t worked Java in doing anything particularly complex (outside of android) in a couple years. I’m having trouble remembering if this is possible:

I’m subclassing ArrayList mainly so I can keep the arraylist ordered. I’m trying to override the add(object) method but I want it to return an int instead of a boolean (the location of the object that was added). But I’m getting errors on the return type of my method.

Is what I want even possible in the language? Can you have a method in a subclass return something different than the superclass’ method?

Or am I trying to do something stupid? Is this breaking the is-a idea of inheritance? Should I just encapsulation an arraylist instead of extending it?

For reference, a portion of what I’m trying to do:

public class AuthorArray extends ArrayList \{

    @Override
    public int add(Author object) {
        super.add(object);

        Collections.sort(this, new SortByLastName());

        return this.indexOf(object);
    }
}

Answer 1

Can you have a method in a subclass return something different than the superclass’ method?

In general, no. The only exception is covariant return types, when an overridden method returns a subclass of the return type in the base class/interface method. This became possible with Java5, and is good practice. But your case does not fall into this category.

Is this breaking the is-a idea of inheritance?

Yes. Users of ArrayList expect to get a boolean return value from add, and see the elements in the same order they added them, and you would break that expectation. Don’t do that.

Should I just encapsulation an arraylist instead of extending it?

Yes. Then you can define your own interface, with whatever contract you prefer. But first, consider using a TreeSet instead.