Utils.java

import java.util.*;

public final class Utils {

    public static List<Integer> flatIntegers(Collection col) {
        if (col == null) {
            throw new IllegalArgumentException("Collection cannot be null");
        }
        List<Integer> result = new ArrayList<>();
        Deque<Iterator> queue = new ArrayDeque<>();
        queue.add(col.iterator());
        while (!queue.isEmpty()) {
            Iterator it = queue.remove();
            while (it.hasNext()) {
                Object element = it.next();
                if (element instanceof Collection) {
                    queue.add(it);
                    it = ((Collection) element).iterator();
                } else if (element instanceof Integer) {
                    result.add((Integer) element);
                } else if (element == null) {
                    throw new IllegalArgumentException("Invalid null element found");
                } else {
                    throw new IllegalArgumentException(
                            "Element must be either a Collection or an integer, but it is a " + element.getClass().getName());
                }
            }
        }
        return result;
    }
}