How to convert list to Map of Map

Issue

I have a code like this:

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), ...);

Could you help me to finish the code base ?

Solution

You could use groupingBy which takes a classifier and a downstream collector. The classifier is used to create the outer map based on division and then the downstream collector (toMap) is used to generate the inner map:

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

Answered By - Gautham M