Question

I am not too new to Java, though I have never really worked with Sets before,
so can anybody please help me out here?

I’m having the following problem;
but first of all, this is my code:

HashMap<Position[], String> save = io.getSave();

Position[][] saved_pos = (Position[][]) save.keySet().toArray();

Though on the second line, Java throws a ClassCastException, but why?
Obviously, the Array returned by save.keySet().toArray() does contain Position[][]s, even though, unfortunately, toArray() in the class Set does only return an Object[] array.

So what can I do, I NEED to cast this.

Answer 1

The toArray() method doesn’t actually know that it’s being called on a set of Position[]s; all it knows is that it’s being called on a set. (This is because of how generics are implemented in Java; not very much information is available at run-time.) So, it just returns an Object[] full of Partition[]s, rather than returning a true Partition[][]. (The difference being that an Object[] can contain arbitrary objects — it would be valid, after your call, to write saved_pos[0] = new Object() — whereas a true Partition[][] would raise a ArrayStoreException if you tried to set one of its elements to new Object().)

Fortunately, there’s a separate toArray(...) method that can be used, where you tell it what array type you want, like so:

Position[][] saved_pos = save.keySet().toArray(new Position[0][]);