Create 12 months rollover data inside a java object

Issue

I have a java object like this Map<Integer, Long> monthCaseCountMap

This object contains data in the format like below :

202103, 2
202104, 4
202105, 0
202106, 9
.
.
.
.
202202, 10

First value is a month key, second value is case count.

I need to create a monthly rollover data or accumulation of case count month by month.

The result should look like -

202103, 2
202104, 6
202105, 6
202106, 15
.
.
.
.
202202, 98

How can we do this in using Java streams or using any other method.

Thank you @eritren for providing the correct solution.

Solution

Another option could be to use AtomicLong to store the sum and collect the accumulated values in to a new map:

Map<Integer, Long> monthCaseCountMap = Map.of(  202103, 2L,
                                                202104, 4L,
                                                202105, 0L,
                                                202106, 9L);

AtomicLong sum = new AtomicLong(0L);

Map<Integer, Long> accumulated = 
        monthCaseCountMap.entrySet()
                .stream()
                .sorted(Map.Entry.comparingByKey())
                .collect(Collectors.toMap(Map.Entry::getKey, e -> sum.addAndGet(e.getValue())));

Answered By - Eritrean