java基础comparable问题

我程序运行返回错误是Exception in thread "main" java.lang.ClassCastException: Item cannot be cast to java.lang.Comparable。
求大神告知解决办法。

public class LinkListTest {
public static void main(String[] args) {
SortedSet 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;
}
}