Java对Collections进行分组

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.mkyong.com/java8/java-8-collectors-groupingby-and-mapping-example/

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