Java8中Collectors.groupingBy的坑，分组乱序

秒速五厘米 2022-10-09 12:09 174阅读 0赞

**先创建个实体**

@Data
    public class User { 
        private String date;
        private String name;
        private String age;
    
        public User(String date, String name, String age) { 
            this.date = date;
            this.name = name;
            this.age = age;
        }
    }

**按日期分下组**

public static void main(String[] args) { 
            List<User> userList = new ArrayList<>();
            for (int x = 1; x < 30; x++) { 
                User user = new User("2021-06-" + (x < 10 ? "0" + x : x), "张三", x);
                userList.add(user);
            }
    
            Map<String, List<User>> userMap = userList.stream().collect(Collectors.groupingBy(User::getDate));
            System.out.println(userMap);
        }

**可以看到分完组的Map顺序是乱的**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM5NDg2MTE5_size_16_color_FFFFFF_t_70]  
**这个方法为什么会乱序，看下源码**

public static <T, K, A, D>
        Collector<T, ?, Map<K, D>> groupingBy(Function<? super T, ? extends K> classifier,
                                              Collector<? super T, A, D> downstream) { 
            return groupingBy(classifier, HashMap::new, downstream);
        }

**可以看到，这个方法默认返回的是HashMap，都知道HashMap是无序的，既然找到问题就好解决了，我们手动传个有序的Map进去就可以了**

public static void main(String[] args) { 
            List<User> userList = new ArrayList<>();
            for (int x = 1; x < 30; x++) { 
                User user = new User("2021-06-" + (x < 10 ? "0" + x : x), "张三", x);
                userList.add(user);
            }
    
            Map<String, List<User>> userMap = userList.stream().collect(Collectors.groupingBy(User::getDate, LinkedHashMap::new, Collectors.toList()));
            System.out.println(userMap);
        }

**可以看到分完组的Map变成有序的了**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM5NDg2MTE5_size_16_color_FFFFFF_t_70 1]

**也可以额外的去除排序**

// 将日期key取出，去重、排序
            List<String> dateList = new ArrayList<>(userMap.keySet());
            dateList = dateList.stream().distinct().collect(Collectors.toList());
            Collections.sort(dateList);

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