Container.java

public class Container implements Iterable<Integer> {
    private final List<Integer> list = new ArrayList<>();

    public void add(int i)
        { list.add(i); }

    public int length()
        { return list.size(); }

    public boolean isEmpty()
        { return list.isEmpty(); }

    @Override public Iterator<Integer> iterator()
        { return list.iterator(); }

    //////////////////////////////////////////////

    public static void main(String... args) {
        Container c;

        //Everytime we want to make sure c contains an instance of Container
        //We are forced to check against null
        if (c != null) c = new Container();
        //Even if we want to represent null as empty
        boolean empty = c == null || c.isEmpty();
    }
}