求大神帮忙写个商品货架管理程序代码并且有保存程序

import java.util.*;

class Untitled {
    static TreeSet<Item> items = new TreeSet<>();
    public Item pop(){
        if(items.isEmpty()){
            return null;
        }
        Item item = items.first();
        items.remove(item);
        return item;
    }
    public void push(Item item){
        items.add(item);
    }
    public static void main(String[] args) {
        Untitled util = new Untitled();
        util.push(new Item("aaa","2014-02-13"));
        util.push(new Item("bbb","2014-03-13"));
        util.push(new Item("ccc","2014-01-23"));
        util.push(new Item("ddd","2014-04-11"));
        System.out.println(items);
        util.pop();
        System.out.println(items);
    }
}
class Item implements Comparable<Item>{
    String name;
    String date;
    public Item(String name, String date){
        this.name=name;
        this.date=date;
    }
    public int compareTo(Item o){
        if(o==null || o.date==null){
            return -1;
        }else if(date==null){
            return 1;
        }
        return (o.date);
    }
    public String toString(){
        return "Item[name = " + name + ", date = " + date + "]";
    }
}