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I have List<Person> where Person is as below.

我有列表 ， Person如下。

class Person {

    String personId;
    LocalDate date;
    String type;

    // getters & setters

}

I'm trying to convert this to List<Person> to Map<String, Map<LocalDate,List<Person>>> where outer map's key is personId and inner map's key is date and I couldn't figure out how to achieve this. Thus far have tried something like below. Open to Java 8 solutions as well.

我试着将它转换为List 映射 >>，外图的键是personId，而inner Map的键是日期，我不知道如何实现这一点。到目前为止，我们已经尝试了如下的方法。也可以打开Java 8解决方案。 ,>

Map<String,Map<LocalDate,List<Person>>> outerMap = new HashMap<>();
Map<LocalDate,List<Person>> innerMap = new HashMap<>();

for(Person p : list) {
    List<Person> innerList = new ArrayList<>();
    innerList.add(p);
    innerMap.put(p.getDate(), innerList);
    outerMap.put(p.getPersonId(), innerMap);
}

3 个解决方案

#1

list.stream()
    .collect(Collectors.groupingBy(
        Person::getPersonId,
        Collectors.groupingBy(
            Person::getDate
)));

#2

This answer shows how to do it with streams and this other one how to do it with a traditional iterative approach. Here's yet another way:

这个答案显示了如何使用流和另一个方法来使用传统的迭代方法。这里有另一种方法:

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

This uses Map.computeIfAbsent to create a new innerMap and put it in the outerMap if not present (the key being personId), and also to create a new innerList and put it in the innerMap if not present (the key being date). Finally, the Person is added to the innerList.

它使用Map.computeIfAbsent创建一个新的innerMap，并将其放入到outerMap中，如果不存在(关键是personId)，还可以创建一个新的innerList，并将其放入到innerMap中，如果不存在(关键是日期)。最后，将Person添加到innerList中。

#3

Your for loop organization should be changed in such a way that it tries getting an existing List<Person> from the nested map before proceeding with creation of the new list. This is a two-step process:

您的for循环组织应该以这样的方式进行更改，以便在继续创建新列表之前，从嵌套映射中获取一个已存在的列表 。这是一个两步的过程:

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);
}