stream分组排序高级使用

java中stream可以对数据集合进行排序，而且还可以指定分组排序，这里罗列出常用的情景。

假设数据集合中的元素是Person，字段的属性如下所示

@Data
public static class Person {
    private Date birthDay;
    private String name;
    private Integer age;
    private String date;

    public Person(Date birthDay, String name, Integer age) {
        this.birthDay = birthDay;
        this.name = name;
        this.age = age;
        this.date = new SimpleDateFormat("yyyy-MM-dd").format(this.birthDay);
    }
}

初始化数据:

    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    List<Person> datas = new ArrayList<>();
    datas.add(new Person(sdf.parse("2020-05-01 12:01:23"), "abc", 10));
    datas.add(new Person(sdf.parse("2020-05-02 12:01:23"), "cde", 99));
    datas.add(new Person(sdf.parse("2020-05-03 12:01:23"), "1234", 8));
    datas.add(new Person(sdf.parse("2020-05-01 12:01:00"), "xxx", 19));
    datas.add(new Person(sdf.parse("2020-05-02 02:01:23"), "111", 13));

场景： 分组排序
按照日期getDate来倒序分组，里面放着生日getBirthDay也是倒序的数据

Map<String, List<Person>> sortedDatas = datas.stream()
        .sorted(Comparator.comparing(Person::getBirthDay, Comparator.nullsLast(Comparator.reverseOrder())))
        .collect(Collectors.groupingBy(Person::getDate, LinkedHashMap::new, Collectors.toList()));

这里先使用getBirthDay全局倒序，然后照着分组getDate分组，这里需要注意的是使用LinkedHashMap不然系统会缺省使用HashMap来构建

打印代码如下

// 打印数据
for (Map.Entry<String, List<Person>> elem : sortedDatas.entrySet()) {
    System.out.println("key:" + elem.getKey());
    for (Person p : elem.getValue()) {
        System.out.println("\t"+p.toString());
    }
}

打印内容

key:2020-05-03
	TestGroupSort.Person(birthDay=Sun May 03 12:01:23 CST 2020, name=1234, age=8, date=2020-05-03)
key:2020-05-02
	TestGroupSort.Person(birthDay=Sat May 02 12:01:23 CST 2020, name=cde, age=99, date=2020-05-02)
	TestGroupSort.Person(birthDay=Sat May 02 02:01:23 CST 2020, name=111, age=13, date=2020-05-02)
key:2020-05-01
	TestGroupSort.Person(birthDay=Fri May 01 12:01:23 CST 2020, name=abc, age=10, date=2020-05-01)
	TestGroupSort.Person(birthDay=Fri May 01 12:01:00 CST 2020, name=xxx, age=19, date=2020-05-01)