如何按年然后按月对日期列表进行分组？

Question

I need your suggestion in the following scenario

I have the following list of dates

def date = new Date()
def dates = [date - 350, date - 400, date - 300, date - 50, date]

I need to get the following map:

[2013:[1:null, 2:5, 3:10,...12:5], 2014:[1:10, 2:30...12:100], ...]

Where year map is: key is month and value is count of dates in that month

I got to group it by year this way

def datesGroupedByYear = dates.groupBy { it[Calendar.YEAR] }

How can I create a map as I described

Thanks for your time

Answer 1

You can add further groups by adding closures to the groupBy call. Calendar's MONTH is off by one (starts at 0), so add one there. Next you want to fill all month with the count, which is the size() of the group by results. This will be collected over 1..12 for each month (proper order and null for the missing groups).

def date = new Date()
def dates = [date-350, date-400, date-300, date-50, date-3, date-2, date]

def result = dates.groupBy{it[Calendar.YEAR]}{it[Calendar.MONTH]+1}.collectEntries{ o->
    [o.key, (1..12).collectEntries{
        [it, o.value.get(it)?.size()] 
    }]
}
println result
//; [2013:[1:null, 2:null, 3:null, 4:null, 5:null, 6:null, 7:null, 8:null, 9:null, 10:1, 11:null, 12:1], 2014:[1:1, 2:null, 3:null, 4:null, 5:null, 6:null, 7:null, 8:null, 9:null, 10:1, 11:3, 12:null]]
assert result[date[Calendar.YEAR]][date[Calendar.MONTH]+1]==3