convert a list of objects to a nested map in java 8

Question

I have a list of objects as follows:

Rating {
Long id;
String aRating;
String bRating;
Date date;
//getters and Setters
}
List<Rating> list = //list with some values in it.

I have to convert the above list to a nested map:

Map<Date, Map<CustomEnum, String>> 
Enum CustomEnum {
 aRatingEnum("aRating", "someValue");
 bRatingEnum("bRating", "someValue");
 }

suppose List contains following values:

Id, Date, aRating, bRating
1 , 2/2/2019, A+, B+
2, 2/2/2018, A, B

then this needs to be converted to following map:

Map = {2/2/2019={aRatingEnum=A+, bRatingEnum=B+}, 2/2/2018 = {aRatingEnum=A, bRatingEnum=B}}

I have tried using java 8:

list.stream().collect(Collectors.groupingBy(x->x.getDate(), Collectors.toMap(
//here I am not able to proceed further how should I approach please help.
)))

• ghufranne: Enum is for the understainding purpose,the code is not copied,

Answer 1

The toMap collects several items into a single collection. What you have is a single object (Rating) and want to create a Map out of it.

I think what you want is collectingAndThen which allows you to group by date and then do the transfer to the Map in the finishing Function.

list.stream()
 .collect(Collectors.collectingAndThen(
 Collectors.groupingBy(x->x.getDate(), 
 lr -> {
 Map<CustomEnum, String> result = new EnumMap<>();
 Rating r = lr.get(0); // take first rating for that date
 result.put(CustomEnum.aRatingEnum, r.getARating());
 result.put(CustomEnum.bRatingEnum, r.getBRating());
 return result;
 })));

Or refactor that lambda into a method, or use Map.of if you're using Java 9 or higher.

This will give you a Map<Date, Map<CustomEnum, String>>.

• ghufranne: but I want only nested Map not list, could it not be possible? moreover thanks for your reponse.
• daniu: @ghufranne The list is created because you might have more than one Rating per date. You'll have to decide how to handle several ratings.
• daniu: @ghufranne not in the object, but you may have several objects for the same date in the list you're streaming. But I made a mistake anyway, I'll edit to let the finisher only use the first object.
• ghufranne: lr.get(0); is wrong, lr would be a map of list, the result of grouping by..