Java Collections.sort() 排序

public class User implements Comparable<User> {

    private String name;
    private int age;
    private int id;

    public User(String name, int age, int id) {
        this.name = name;
        this.age = age;
        this.id = id;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "User{" +
                "name=‘" + name + ‘\‘‘ +
                ", age=" + age +
                ", id=" + id +
                ‘}‘;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public User() {
    }
    
    // 重写compareTo方法
    @Override
    public int compareTo(User o) {
        // 先比较age
        int i = this.getAge() - o.getAge();
        if (i == 0) {
            //age相等，再比较id
            int j = this.getId() - o.getId();
            return j;
        }
        return i;
    }
}

public class Test {
    public static void main(String[] args) {

        List<User> list = new ArrayList<>();
        list.add(new User("张三", 19, 4));
        list.add(new User("李四", 18, 2));
        //Collections.addAll(list, new User("王五", 17, 3), new User("赵六", 17, 1));
        list.add(new User("王五", 17, 3));
        list.add(new User("赵六", 17, 1));

        System.out.println("排序前：" + list);
        Collections.sort(list);
        System.out.println("排序后：" + list);
    }
}

public class Test2 {

    public static void main(String[] args) {

        List<String> list = new ArrayList<String>();
        list.add("aa");
        list.add("cc");
        list.add("bb");

//        1、o1大于o2，返回正整数
//        2、o1等于o2，返回 0
//        3、o1小于o3，返回负整数

        Collections.sort(list, new Comparator<String>() {
            @Override
            public int compare(String o1, String o2) {
                //return o1.compareTo(o2); // 升序
                return o2.compareTo(o1); // 降序
            }
        });

        System.out.println(list);  // [cc, bb, aa]
    }
}