java基础问题，求各位大神帮忙解答，感激不尽！报错

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程序运行返回Exception in thread "main" java.lang.ClassCastException: Item cannot be cast to java.lang.Comparable.
求各位大哥大叔帮看看哪里出了问题。。。。
public class LinkListTest {

public static void main(String[] args) { SortedSet<Item> oo = new TreeSet<>(); oo.add(new Item("afang", 1011)); oo.add(new Item("fangjie", 1222)); oo.add(new Item("fangfang", 889)); System.out.println(oo);

SortedSet<Item> sortedByDes = new TreeSet<>(new 
        Comparator<Item>() {
        public int compare(Item a, Item b) {
        String desA = a.getDescription();
        String desB = b.getDescription();
        return desA.compareTo(desB);
        }
});
sortedByDes.addAll(oo);
System.out.println(sortedByDes);

}

} class Item { private String description; private int id; public Item(String aDes, int aId) { description = aDes; id = aId; }

public String getDescription() {
    return description;
}

}