Casting between ArrayLists in Java

Question

Sorry, I thought this was an inheritance question: it was an ArrayList question all along!

Ok, my problem is more specific than I thought. So I have two families of

Answer 1

Marc and kolstae have given good answers in terms of how to get around the problem, but I think it's worth explaining why your original code doesn't work.

To simplify matters, I tend to put the problem in terms of fruit. Suppose we have:

List bananaBunch = new ArrayList();
List fruitBowl = bananBunch;
fruitBowl.add(new Apple());

If this is allowed, we end up with an apple in a bunch of bananas, which is obviously a bad thing. So, where is the problem? The first line has to be okay, and the third line has to be okay - you can add any kind of fruit to List - so the problem has to be in the second line. That's what's prohibited, precisely to avoid this kind of issue.

Does that help at all?