使用TreeSet保存自定义对象时，

必须让定义对象的类实现Comparable接口,并重写compareTo()方法

否则报

实体类User：cannot be cast to java.lang.Comparable异常

public class User implements Comparable
     
      {    private int age;    private String name;    private String pro;    public User(String name,int age,String pro) {        this.name = name;        this.pro = pro;        this.age = age;    }        @Override    public int compareTo(User u) {        if(this.age>u.age) {            return 1;        }else if(this.age==u.age) {            return this.name.compareTo(u.name);        }else {            return -1;        }            }    public int getAge() {        return age;    }    public void setAge(int age) {        this.age = age;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public String getPro() {        return pro;    }    public void setPro(String pro) {        this.pro = pro;    }}
     

测试类Test

public class test{    public static void main(String[] args) {        // TODO Auto-generated method stub        Set
     
       set = new    TreeSet
      
       ();        User user = new User("Java", 21,"小仙女");        User user2 = new User("Spring",22,"小跟班");        set.add(user);        set.add(user2);        System.out.println(set.size());        //遍历方式一//        Iterator
       
         it = set.iterator();//        while(it.hasNext()) {//            User u = it.next(); //            System.out.println(u.getName());//        }        for (User u : set) {            System.out.println(u.getName()+","+u.getAge()+","+u.getPro());        }    }}