public Set<T> getSelectedItems() {
        final Set<T> selectedItems = new HashSet<>();
        for (final T tickBox : items) {
            if (tickBox.getBooleanValue()) {
                selectedItems.add(tickBox);
            }
        }
        return selectedItems;
    }
Handle the case when the list gets populated after the selected items
have been set.
stroom-core-client-widget/src/main/java/stroom/widget/tickbox/client/view/TickList.java