Best way to cast Iterator<Object> to a Set<String>, for instance

Question

Casting an Iterator<Object> to a Set<String>

What would be the cleanest/best practice way?

Answer 1

public Set<B> getBs(){
    Iterator<A> iterator = myFunc.iterator();
    Set<B> result = new HashSet<B>();
    while (iterator.hasNext()) {
        result.add((B) iterator.next();
    }
    return result;
}

But of course, it will fail if all the As returned by the iterator are not Bs.

If you want to filter the iterator, then use instanceof:

public Set<B> getBs(){
    Iterator<A> iterator = myFunc.iterator();
    Set<B> result = new HashSet<B>();
    while (iterator.hasNext()) {
        A a = iterator.next();
        if (a instanceof B) {
            result.add((B) iterator.next();
        }
    }
    return result;
}

Using Guava, the above can be reduced to

return Sets.newHashSet(Iterators.filter(myFunc.iterator(), B.class));

Answer 2

I'm still not 100% sure what you want, but check this out and see:

public static void main(String[] args) {
  final Iterator<?> it = Arrays.asList(new Object[] {"a", "b", "c"}).iterator();
  System.out.println(setFromIterator(it));
}

public static Set<String> setFromIterator(Iterator<?> it) {
  final Set<String> s = new HashSet<String>();
  while (it.hasNext()) s.add(it.next().toString());
  return s;
}