Iterate over non-null portion of array in class implementing Iterable<Item>

Question

I am experienced in other languages but very new to Java. As an optimisation for a specific situation, I'm trying to implement an iterable container class from an array, as I know my container will hold a maximum of 4 items at a time, and those items each belong in a specific index, not first in gets the lowest index available.

To save me checking for null when I iterate over the list, I only want the non-null values returned for iteration... Would the below override of 'iterator()' work? And is there any chance the garbage collector going to clear up the iterable list I return before I get to iterate over it? Or is there a better way to achieve this maybe?

class FixedArray<T> implements Iterable<T> {

    FixedArray() {}

    public void add(byte index, T item) {
        array[index] = item;
    }

    @Override
    public Iterator<T> iterator() {
        List<T> listWithoutNull = new ArrayList<>();
        for (Item item: array) {
            if (item != null) {
                listWithoutNull.add(item);
            }
        }
        return listWithoutNull.iterator();
    }

    private final T[] array = { null, null, null, null };
}

Answer 1

Would the below override of 'iterator()' work?

Yes, why not? Overriding iterator() is perfectly fine. However, this is not an efficient implementation. I would write something like this:

@Override
public Iterator<T> iterator() {
    return Arrays.stream(array).filter(Objects::nonNull).iterator();
}

Here, no intermediate collection will be created, so no overhead.

And is there any chance the garbage collector going to clear up the iterable list I return before I get to iterate over it?

No, the garbage collector is not allowed to do this while you are holding a strong reference to the Iterator object (unless your Iterator implementation uses something like weak references inside but this is not your case).