Java对Collections进行分组

爱被打了一巴掌 2022-06-08 05:22 190阅读 0赞

在批量处理数据的时候，想对一批数据（`List<Object>`）分组操作咋办，有什么简便的方法吗？

准备一点数据：

public class User {
        private Integer id;
        private String type;
        private String name;
        private String date;
        private BigDecimal money;
        private Integer isOk;
    
        public User(Integer id, String name, String type, String date, BigDecimal money, Integer isOk){
            this.id = id;
            this.type = type;
            this.name = name;
            this.date = date;
            this.money = money;
            this.isOk = isOk;
        }
    
        // omit setter and getter
        @Override
        public String toString() {
            return "User{" +
                    "id=" + id +
                    ", type='" + type + '\'' +
                    ", name='" + name + '\'' +
                    ", date='" + date + '\'' +
                    ", money=" + money +
                    ", isOk=" + isOk +
                    '}';
        }
    }
    
     User user1 = new User(1,"张三","小学", "2017-03", BigDecimal.valueOf(123.44), 1);
     User user2 = new User(2,"李四","小学", "2017-03", BigDecimal.valueOf(234.45), 0);
     User user3 = new User(3,"王五","初中", "2017-02", BigDecimal.valueOf(234.12), 1);
     User user4 = new User(4,"马六","高中", "2017-04", BigDecimal.valueOf(123), 0);
     User user5= new User(5,"jack","大学", "2017-04", BigDecimal.valueOf(12), 0);

现在想对用户列表根据日期进行分组，得到一个map: `date -> List<User>`，Java7以前也许是这样的：

List<User> userList = Arrays.asList(user1, user2, user3, user5, user4);
            Map<String, List<User>> groupUserMap = Maps.newHashMap();
            for (User user : userList) {
                String month = user.getDate();
                if (groupUserMap.containsKey(month)) {
                    groupUserMap.get(month).add(user);
                } else {
                    groupUserMap.put(month, Lists.newArrayList(user));
                }
            }

可是有了Java8之后，一切似乎变简单了：

// one line 
    Map<String, List<User>> groupUserMapOne =userList.stream().collect(Collectors.groupingBy(User::getDate));

一行代码搞定。

如果想分组的List里是User的name呢：

// if you want a map: date -> user's names
            Map<String, List<String>> userByDate = Stream.of(user1, user2, user3, user5, user4).collect(Collectors.groupingBy(User::getDate,
                    Collectors.mapping(User::getName, Collectors.toList())));
            System.out.println(userByDate);

运行结果：  
`{2017-03=[张三, 李四], 2017-04=[jack, 马六], 2017-02=[王五]}`

参考：  
[https://www.leveluplunch.com/java/tutorials/039-group-objects-by-field-java8/][https_www.leveluplunch.com_java_tutorials_039-group-objects-by-field-java8]  
[https://www.mkyong.com/java8/java-8-collectors-groupingby-and-mapping-example/][https_www.mkyong.com_java8_java-8-collectors-groupingby-and-mapping-example]

[https_www.leveluplunch.com_java_tutorials_039-group-objects-by-field-java8]: https://www.leveluplunch.com/java/tutorials/039-group-objects-by-field-java8/
[https_www.mkyong.com_java8_java-8-collectors-groupingby-and-mapping-example]: https://www.mkyong.com/java8/java-8-collectors-groupingby-and-mapping-example/