Java Stream对List分组、排序、组内排序

2023-11-22 06:22| 来源: 网络整理| 查看: 265

项目场景：

前段时间遇到了一个业务场景，要对List对象列表进行较复杂的排序操作：首先要对列表根据对象属性A进行分组，然后要对分组后的每组内的对象属性B（每组的属性B值相同，可能为空）对组进行排序，然后每组组内要对属性C进行排序

这里的对象为People类属性为：

@Data public class People { /**国家*/ private String country; /**国庆*/ private Date nationalDay; /**年龄*/ private Integer age; /**姓名*/ private String name; }

即原始数据如下：

[People{country='Egypt', nationalDay=1952-07-23, age=27, name='工具人0'} , People{country='China', nationalDay=1949-10-01, age=28, name='工具人1'} , People{country='Japan', nationalDay=null, age=25, name='工具人2'} , People{country='Korea', nationalDay=1948-08-15, age=23, name='工具人3'} , People{country='Egypt', nationalDay=1952-07-23, age=27, name='工具人4'} , People{country='Japan', nationalDay=null, age=24, name='工具人5'} , People{country='Japan', nationalDay=null, age=25, name='工具人6'} , People{country='U.S', nationalDay=1776-07-04, age=22, name='工具人7'} , People{country='Egypt', nationalDay=1952-07-23, age=23, name='工具人8'} , People{country='China', nationalDay=1949-10-01, age=20, name='工具人9'} , People{country='Egypt', nationalDay=1952-07-23, age=27, name='工具人10'} , People{country='Korea', nationalDay=1948-08-15, age=20, name='工具人11'} , People{country='Korea', nationalDay=1948-08-15, age=21, name='工具人12'} , People{country='Egypt', nationalDay=1952-07-23, age=22, name='工具人13'} , People{country='Korea', nationalDay=1948-08-15, age=25, name='工具人14'} ]

目标数据为：即先对对象的country进行分组，然后用nationalDay对每组进行倒序（nationalDay属性为空放最后），每组组内对age进行升序

[People{country='Egypt', nationalDay=1952-07-23, age=22, name='工具人13'} , People{country='Egypt', nationalDay=1952-07-23, age=23, name='工具人8'} , People{country='Egypt', nationalDay=1952-07-23, age=27, name='工具人0'} , People{country='Egypt', nationalDay=1952-07-23, age=27, name='工具人4'} , People{country='Egypt', nationalDay=1952-07-23, age=27, name='工具人10'} , People{country='China', nationalDay=1949-10-01, age=20, name='工具人9'} , People{country='China', nationalDay=1949-10-01, age=28, name='工具人1'} , People{country='Korea', nationalDay=1948-08-15, age=20, name='工具人11'} , People{country='Korea', nationalDay=1948-08-15, age=21, name='工具人12'} , People{country='Korea', nationalDay=1948-08-15, age=23, name='工具人3'} , People{country='Korea', nationalDay=1948-08-15, age=25, name='工具人14'} , People{country='U.S', nationalDay=1776-07-04, age=22, name='工具人7'} , People{country='Japan', nationalDay=null, age=24, name='工具人5'} , People{country='Japan', nationalDay=null, age=25, name='工具人2'} , People{country='Japan', nationalDay=null, age=25, name='工具人6'} ]

解决方案：

我的方案是通过Stream对列表进行分组排序等操作达到预期结果：

1、 country分组

分组为Map集合key-value值为country属性-同country属性的列表List

// 1、country分组 Map countryGroupMap = peopleList.stream().collect(Collectors.groupingBy(People::getCountry)); System.err.println(countryGroupMap);

2、 country分组每组取一条 // country分组每组取一条 Map countryGroupGetOneMap = peopleList.stream().collect( Collectors.groupingBy( People::getCountry,Collectors.collectingAndThen( Collectors.reducing((c1, c2) -> c1), Optional::get) ) ); System.err.println(countryGroupGetOneMap);

3、对分组后的map根据nationalDay倒序（birthday可能为空null放最后）

注意：用LinkedHashMap保存，因为HashMap是根据key计算hash值设置元素存储位置的是无序的，故应用有序map保存排序后数据

// 对分组后的map根据nationalDay倒序（birthday可能为空null放最后） Map linkedHashMap = new LinkedHashMap(); countryGroupGetOneMap.entrySet().stream().sorted( Map.Entry.comparingByValue( Comparator.comparing( People::getNationalDay, Comparator.nullsFirst(Date::compareTo) ) ).reversed() ).forEachOrdered(e -> linkedHashMap.put(e.getKey(), e.getValue())); System.err.println(linkedHashMap);

4、获取根据 nationalDay 排序后的 country列表 // 获取根据 nationalDay 排序后的 country列表 List birthdaySortCountryGroupCountryList = new ArrayList(linkedHashMap.keySet()); 5、根据country列表获取完整分组、排序、排序类表（每组再根据年龄升序）得到最终结果 // 根据country列表获取完整分组、排序、排序类表（每组再根据年龄升序） List finalPeopleGroupSortList = new ArrayList(); for (String country : nationalDaySortCountryGroupCountryList) { // 每组再根据年龄升序 List ageSortGroup = countryGroupMap.get(country).stream().sorted(Comparator.comparing(People::getAge)).collect(Collectors.toList()); finalPeopleGroupSortList.addAll(ageSortGroup); } System.err.println(finalPeopleGroupSortList);

附上完整代码：

/** * @author zg * @create 2021/8/24 19:19 */ public class GroupTanSortExample { public static void main(String[] args) { // 生成乱序的People对象列表 List peopleList = GroupTanSortExample.randomPeopleList(15); System.err.println(peopleList); // 1、country分组 Map countryGroupMap = peopleList.stream().collect(Collectors.groupingBy(People::getCountry)); System.err.println(countryGroupMap); // 2、country分组每组取一条 Map countryGroupGetOneMap = peopleList.stream().collect( Collectors.groupingBy( People::getCountry,Collectors.collectingAndThen( Collectors.reducing((c1, c2) -> c1), Optional::get) ) ); System.err.println(countryGroupGetOneMap); // 3、对分组后的map根据nationalDay倒序（nationalDay可能为空null放最后） Map linkedHashMap = new LinkedHashMap(); countryGroupGetOneMap.entrySet().stream().sorted( Map.Entry.comparingByValue( Comparator.comparing( People::getNationalDay, Comparator.nullsFirst(Date::compareTo) ) ).reversed() ).forEachOrdered(e -> linkedHashMap.put(e.getKey(), e.getValue())); System.err.println(linkedHashMap); // 4、获取根据 nationalDay 排序后的 country列表 List nationalDaySortCountryGroupCountryList = new ArrayList(linkedHashMap.keySet()); // 5、根据country列表获取完整分组、排序、排序类表（每组再根据年龄升序） List finalPeopleGroupSortList = new ArrayList(); for (String country : nationalDaySortCountryGroupCountryList) { // 每组再根据年龄升序 List ageSortGroup = countryGroupMap.get(country).stream().sorted(Comparator.comparing(People::getAge)).collect(Collectors.toList()); finalPeopleGroupSortList.addAll(ageSortGroup); } System.err.println(finalPeopleGroupSortList); } // 生成乱序的People对象 public static List randomPeopleList(int size) { List list = new ArrayList(); for (int i = 0; i < size; i++) { People people = new People(); people.setName("工具人" + i); int num = (int) (Math.random() * 10 % 5); people.setAge(20 + (int) (Math.random() * 10 % 9)); switch (num) { case 0:people.setCountry("China");people.setNationalDay(parserDate("1949-10-01 00:00:00"));break; case 1:people.setCountry("Japan");people.setNationalDay(null);break; case 2:people.setCountry("U.S");people.setNationalDay(parserDate("1776-07-04 00:00:00"));break; case 3:people.setCountry("Korea");people.setNationalDay(parserDate("1948-08-15 00:00:00"));break; case 4:people.setCountry("Egypt");people.setNationalDay(parserDate("1952-07-23 00:00:00"));break; default:; } list.add(people); } return list; } // 字符串解析为日期 public static Date parserDate(String dateStr) { Date date = null; try { date = new SimpleDateFormat("yyy-MM-dd HH:mm:mm").parse(dateStr); } catch (ParseException e) { e.printStackTrace(); } return date; } }

感觉目标是达到了，但是代码还是太冗长，Stream肯定还有更方便的方法，期待更简洁更优雅的实现方式

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