Java 将List＜Object＞ 转换成 Map＜String, Map＜String,List＜Object＞＞＞的几种方法

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);
}