Subtyping or Generics in Java?

Question

This is a Java Assignment question, possibly concerning sub-typing and generics. I have a class that extends ArrayList called Rows

public class Rows extends ArrayList<List<Thing>> implements List<List<Thing>>{}

I need to return it in another class which has a method that returns a List<List<Thing>>, thus I need to do this in order to get the desired return type:

private List<List<Thing>> list;

public List<List<Thing>> rows() {
    Rows r = (Rows) list;// But this cast does not work at runtime
    return (List<List<Thing>>) r;
}

The error Eclipse returns is java.lang.ClassCastException: java.util.ArrayList cannot be cast to package.Rows. I am quite confused as my Rows class extends ArrayList and I thought that it should be able to be cast to it.

Answer 1

Rows extends ArrayList<List<Thing>>

The above definition says that Rows is a subclass of ArrayList<List<Thing>> (which means it's a specialized version), not that Rows is the same thing as an ArrayList<List<Thing>>.

This means that a Rows is a kind of ArrayList<List<Thing>> (so upcasting works) but an ArrayList<List<Thing>> is not necessarily a Rows (so downcasting doesn't work).

If you have an object that you've created as a new ArrayList<List<Thing>>, then this is its type and you can't downcast it further. If you want to be able to use it as a Rows, you simply need to create it it as a new Rows.

(By the way, plural is unconventional for class names.)