示例Person类
class Person { String personId; LocalDate date; String type; // getters & setters }
1、使用stream()进行转换
list.stream()
.collect(Collectors.groupingBy(
Person::getPersonId,
Collectors.groupingBy(
Person::getDate
)));
2、使用foreach实现转换
Map<String, Map<LocalDate, List<Person>>> outerMap = new HashMap<>(); list.forEach(p -> outerMap .computeIfAbsent(p.getPersonId(), k -> new HashMap<>()) // returns innerMap .computeIfAbsent(p.getDate(), k -> new ArrayList<>()) // returns innerList .add(p)); // adds Person to innerList
3、使用for循环转换
Map<String,Map<LocalDate,List<Person>>> outerMap = new HashMap<>();
for(Person p : list) {
Map<LocalDate,List<Person>> innerMap = outerMap.get(p.getPersonId());
if (innerMap == null) {
innerMap = new HashMap<>();
outerMap.put(p.getPersonId(), innerMap);
}
List<Person> innerList = innerMap.get(p.getDate());
if (innerList == null) {
innerList = new ArrayList<>();
innerMap.put(p.getDate(), innerList);
}
innerList.add(p);
}