Java : I have a list of String which holds Date column data from database. I want the count of each month .

Say I have data : 2021-07-01 2021-08-20 2021-08-25 2021-09-05 2022-01-06 2022-06-07

I want to retrieve value 1 for July,2 for August, 1 for September, 1 for January, 1 for June .

How can I use stream API to achieve this? Or any other method to retrieve vthis data

CodePudding user response：

I wrote up a process to allow you to get your desired goal using Streams and Lambdas.

1. Parse your dates into an Array of LocalDates.

2. Define a Predicate for your criteria.

3. Filter your Stream using your Predicate and get the count().

    String[] dates = { "2021-07-01", "2021-08-20", "2021-08-25", "2021-09-05", "2022-01-06", "2022-06-07" };
    LocalDate[] localDates = new LocalDate[dates.length];
   
    for(int i = 0; i < dates.length; i  ){
        localDates[i] = LocalDate.parse(dates[i]);
    }
   
    Predicate<LocalDate> month = localDate -> localDate.getMonth().equals(Month.AUGUST);
   
    long count = Arrays.stream(localDates).filter(month).count();
   

CodePudding user response：

Make example data.

List< String > inputs = List.of( "2021-07-01", "2021-08-20", "2021-08-25", "2021-09-05", "2022-01-06", "2022-06-07" );

Make a stream of those inputs. Parse each element as a LocalDate object, for a new stream. Produce a key-value Map < YearMonth , Integer > object by extracting a YearMonth from each LocalDate as the key, and summing one as a count of occurrences for the value.

Map< YearMonth , Integer > monthCounts = 
    inputs
        .stream()
        .map( input -> LocalDate.parse( input ) )              // Parse each element as a `LocalDate` object, for a new stream.
        .collect(
            Collectors.groupingBy( date -> 
                           YearMonth.from( date ) ,            // Extract a `YearMonth` from each `LocalDate` as the key.
                           Collectors.summingInt( date -> 1 )  // Count occurrences by adding one. 
            )
        )                                                      // Return a `Map`.
;

See this code run live at Ideone.com.

{2021-09=1, 2021-08=2, 2021-07=1, 2022-06=1, 2022-01=1}

I imagine there is a better way to count occurrences than Collectors.summingInt, but I don’t know.