Question

In a piece of code I’ve written, I have this line:

         AllSprites = (ArrayList<ClSprite>) savedInstanceState.getParcelableArrayList("AllSprites");

I’m getting an error about an invalid cast from an ArrayList<Parcelable> to ArrayList<ClSprite>. Why isn’t this legal?

Answer 1

It is fundamentally unsafe to cast an ArrayList<Derived> to an ArrayList<Base> or vice-versa. Doing so opens up a hole in the type system, and Java will throw a ClassCastException at runtime if you try this.

The reason is that I could do something like this:

ArrayList<Derived> derived = new ArrayList<Derived>();
ArrayList<Base> base = (ArrayList<Derived>) derived; // Not legal!
base.add(new Base()); // Just put a Base into the list, but it only holds Derived!
derived.get(0).doSomethingOnlyInDerived(); // Error!  It's not really a Derived!

This is the reason, by the way, that Java’s implicit conversions between arrays are broken and why there’s ArrayStoreException. This cast isn’t safe under all cases.