java中stream可以對數據集合進行排序,而且還可以指定分組排序,這裏羅列出常用的情景。
假設數據集合中的元素是Person
,字段的屬性如下所示
@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);
}
}
初始化數據:
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));
場景: 分組排序
按照日期getDate來倒序分組,裏面放着生日getBirthDay
也是倒序的數據
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()));
這裏先使用getBirthDay全局倒序,然後照着分組getDate分組,這裏需要注意的是使用LinkedHashMap
不然系統會缺省使用HashMap
來構建
打印代碼如下
// 打印數據
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());
}
}
打印內容
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)