如何从可以属于两个或多个组的列表中对对象进行分组? [英] How to group objects from a list which can belong to two or more groups?
问题描述
我有一个项目列表,其中每个项目都可以属于一个或多个类别.对于一组有限的category(string),我想创建一个类别为键,Items列表为值的地图.
I have a list of Items where each Item can belong to one or more category. For a limited set of categories(string) I want to create a map with category as key and list of Items as value.
假设我的Item类的定义如下所示:
Assume my Item class is defined as shown below:
public static class Item{
long id;
List<String> belongsToCategories;
public List<String> getBelongsToCategories() {
return belongsToCategories;
}
public void setBelongsToCategories(List<String> belongsToCategories) {
this.belongsToCategories = belongsToCategories;
}
public Item(long id,List<String> belongsToCategories) {
this.id = id;
this.belongsToCategories = belongsToCategories;
}
@Override
public String toString() {
return "Item{" + "id=" + id + '}';
}
}
和项目列表:
public static void main(String[] args) {
List<Item> myItemList = new ArrayList<>();
myItemList.add(new Item(1,Arrays.asList("A","B")));
myItemList.add(new Item(2,Arrays.asList("A","C")));
myItemList.add(new Item(3,Arrays.asList("B","C")));
myItemList.add(new Item(4,Arrays.asList("D")));
myItemList.add(new Item(5,Arrays.asList("D","E")));
myItemList.add(new Item(6,Arrays.asList("A","F")));
Map<String,List<Item>> myMap= new HashMap<>();
如何从myList填充myMap?
How can i fill myMap from myList?
我认为Stream API可以提供帮助,但是当某项可以属于一个或多个类别时,我不知道将哪个分类器放入groupingBy方法中
I thought Stream API could help, but I don't know which classifier to put in the groupingBy method when an Item can belong to one or more categories
myItemList.stream().collect(Collectors.groupingBy(classifier));
此
myItemList.stream().collect(Collectors.groupingBy(Item::getBelongsToCategories));
产生
[D, E]=[Item{id=5}]
[B, C]=[Item{id=3}]
[A, B]=[Item{id=1}]
[D]=[Item{id=4}]
[A, C]=[Item{id=2}]
[A, F]=[Item{id=6}]
预期很难,例如:
A=[Item{id=1}, Item{id=2}, Item{id=6}]
B=[Item{id=1}, Item{id=3}]
C=[Item{id=2}, Item{id=3}]
D=[Item{id=4}, Item{id=5}]
E=[Item{id=5}]
F=[Item{id=6}]
推荐答案
您可以使用flatMap
映射到SimpleEntry
,然后将groupingBy
映射为:
You can use flatMap
to map to SimpleEntry
and then groupingBy
as :
return items.stream()
.flatMap(p -> p.getBelongsToCategories()
.stream()
.map(l -> new AbstractMap.SimpleEntry<>(l, p)))
.collect(Collectors.groupingBy(Map.Entry::getKey,
Collectors.mapping(Map.Entry::getValue,
Collectors.toList())));
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