List排序

前言：List排序

基础类型排序

对基础类型（包装类）的排序

List<Long> list = new ArrayList<>();
for (int i = 0; i < 10; i++) {
    list.add(Math.round(Math.random()*10));
}
// 升序
Collections.sort(list);
// 降序
Collections.sort(list,Collections.reverseOrder());
for (Long i : list) {
    System.out.println(i);
}

对象排序

第一种方法，对象继承Comparable接口并重写compareTo方法

public class Main {
    public static void main(String[] args) {
        List<User> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            list.add(new User(i));
        }
        Collections.sort(list);
        for (User user : list) {
            System.out.println(user.id);
        }
    }

    static class User implements Comparable {
        int id;

        public User() {
        }

        public User(int id) {
            this.id = id;
        }
        // 升序：this > o,返回1;相等返回0;this < o,返回-1
        // 降序：this > o,返回-1;相等返回0;this < o,返回1
        @Override
        public int compareTo(Object o) {
            User user = (User) o;
            if (id > user.id) {
                return 1;
            } else if (id == user.id) {
                return 0;
            } else {
                return -1;
            }
        }
    }
}

第二种方法，在Collections.sort方法中重写compare方法

public class Main {
    public static void main(String[] args) {
        List<User> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            list.add(new User(i));
        }
        Collections.sort(list, new Comparator<User>() {
            // 升序：o1 > o2,返回1;相等返回0;o1 < o2,返回-1
            // 降序：o1 > o2,返回-1;相等返回0;o1 < o2,返回1
            @Override
            public int compare(User o1, User o2) {
                if (o1.id > o2.id) {
                    return 1;
                } else if (o1.id == o2.id) {
                    return 0;
                } else {
                    return -1;
                }
            }
        });
        for (User user : list) {
            System.out.println(user.id);
        }
    }

    static class User {
        private int id;

        public User() {
        }

        public User(int id) {
            this.id = id;
        }
        
    }
}

List<Map<String,String>>排序

// 一定要注意字符串比较即compareTo方法排序规则有特殊之处即 "111"小于"22"
Collections.sort(listName, new Comparator<Map<String, String>>() {
    public int compare(Map<String, String> o1, Map<String, String> o2) {
        return o1.get("key").compareTo(o2.get("key"));
    }
});

compareTo方法

public int compareTo(String anotherString) {
    int len1 = value.length;
    int len2 = anotherString.value.length;
    int lim = Math.min(len1, len2);
    char v1[] = value;
    char v2[] = anotherString.value;

    int k = 0;
    while (k < lim) {
        char c1 = v1[k];
        char c2 = v2[k];
        if (c1 != c2) {
            return c1 - c2;
        }
        k++;
    }
    return len1 - len2;
}