public abstract class ListItem {
    protected ListItem rightItem = null;
    protected ListItem leftItem = null;

    protected Object value;

    public ListItem(Object value) {
        this.value = value;
    }

    abstract ListItem next();
    abstract ListItem setNext(ListItem item);
    abstract ListItem previous();
    abstract ListItem setPrevious(ListItem item);

    abstract int compareTo(ListItem item);

    public Object getValue() {
        return value;
    }

    public void setValue(Object value) {
        this.value = value;
    }
}



public class Node extends ListItem {

    public Node(Object value) {
        super(value);
    }

    @Override
    ListItem next() {
        return this.rightItem;
    }

    @Override
    ListItem setNext(ListItem item) {
        this.rightItem = item;
        return this.rightItem;
    }

    @Override
    ListItem previous() {
        return this.leftItem;
    }

    @Override
    ListItem setPrevious(ListItem item) {
        this.leftItem = item;
        return this.leftItem;
    }

    @Override
    int compareTo(ListItem item) {
        if(item != null) {
            return ((String) super.getValue()).compareTo((String) item.getValue());
        } else {
            return -1;
        }
    }
}