比较器 Comparable 与compartor 的区别及理解

public static void compareTest(){
    List<User> list = new ArrayList<User>();
    list.add(new User("bob",27));
    list.add(new User("club",26));
    Collections.sort(list, new Comparator<User>() {
        public int compare(User o1, User o2) {
            return o1.getAge()-o2.getAge();
        }
    });
    //[User{name=‘club‘, age=26}, User{name=‘bob‘, age=27}]
    System.out.println(list.toString());
}