JavaStreamMaxMinAverage

// Data Model
public class DataModel {
    private LocalDate date;
    private double initial;
    private double peak;
    private double basal;
    private double end;

    public DataModel(LocalDate date, double initial, double peak, double basal, double end) {
        this.date = date;
        this.initial = initial;
        this.peak = peak;
        this.basal = basal;
        this.end = end;
    }

    public LocalDate getDate() {
        return date;
    }

    public double getInitial() {
        return initial;
    }

    public double getPeak() {
        return peak;
    }

    public double getBasal() {
        return basal;
    }

    public double getEnd() {
        return end;
    }
}

// Main Class
public class main {
    public static void main(String[] args) {
        List<DataModel> list = getData();

        Map<String, TemporalAdjuster> ADJUSTERS = new HashMap<>();
        ADJUSTERS.put("day", TemporalAdjusters.ofDateAdjuster(d -> d)); // identity
        ADJUSTERS.put("week", TemporalAdjusters.previousOrSame(DayOfWeek.of(1)));
        ADJUSTERS.put("month", TemporalAdjusters.firstDayOfMonth());
        ADJUSTERS.put("year", TemporalAdjusters.firstDayOfYear());

        Map<Object, List<DataModel>> temp = list
                .stream()
                .collect(Collectors.groupingBy((DataModel data) -> data.getDate().with(ADJUSTERS.get("week"))));

        System.out.println(temp.size());

    }

    private static List<DataModel> getData(){
        List<DataModel> list = new ArrayList<>();
        list.add(new DataModel(LocalDate.of(2018,12,9),135,141,106,116));
        list.add(new DataModel(LocalDate.of(2018,12,8),114,188,111,134));
        list.add(new DataModel(LocalDate.of(2018,12,7),161,170,130,146));
        list.add(new DataModel(LocalDate.of(2018,12,6),161,192,117,127));
        list.add(new DataModel(LocalDate.of(2018,12,5),154,190,110,165));
        list.add(new DataModel(LocalDate.of(2018,12,4),140,186,100,134));
        list.add(new DataModel(LocalDate.of(2018,12,3),126,183,165,145));
        list.add(new DataModel(LocalDate.of(2018,12,2),111,143,109,119));
        list.add(new DataModel(LocalDate.of(2018,12,1),138,188,122,162));
        list.add(new DataModel(LocalDate.of(2018,11,30),112,147,116,127));
        list.add(new DataModel(LocalDate.of(2018,11,29),162,168,154,163));
        list.add(new DataModel(LocalDate.of(2018,11,28),138,181,101,151));
        list.add(new DataModel(LocalDate.of(2018,11,27),142,176,102,159));
        list.add(new DataModel(LocalDate.of(2018,11,26),134,153,117,120));
        return list;
    }
}