如何将列表转换为地图的地图

Question

我有这样一个代码：

class MyObject{
    String division;
    String subDivision;
    Integer size;

    public MyObject(String division, String subDivision, Integer size) {
        this.division = division;
        this.subDivision = subDivision;
        this.size = size;
}

List<MyObject> list = Arrays.asList(new MyObject("A", "AA", 4),
        new MyObject("A", "AB", 2),
        new MyObject("A", "AC", 3),
        new MyObject("B", "BA", 11),
        new MyObject("B", "BB", 7),
        new MyObject("C", "CA", 8));
Map<String, Map<String,Integer>> map
    = list.stream().collect(Collectors.toMap(MyObject::getDivision), ...);

你能帮我完成代码库吗？

Answer 1

您可以使用groupingby，它接受分类器和下游收集器。分类器用于基于division创建外部映射，然后使用下游收集器(tomap)生成内部映射：

list.stream()
    .collect(Collectors.groupingBy(MyObject::getDivision,
                                   Collectors.toMap(MyObject::getSubDivision, MyObject::getSize)));