java - Check if dates are overlapping and return the maximum count

Question

I've multiple dates with start and end. These dates could be like follows:

d1:      |----------|
d2:            |------|
d3:        |--------------|
d4:                         |----|
d5:   |----|

Now I need to check the maximum count of overlapping dates. So in this example, we got maximum 3 overlapping dates (d1, d2, d3). Consider, that there can be 0 to n dates.

Can you help me with this task? Thank you in advance.

UPDATE

Input: List of Java-Dates with start and end point, for example List, where MyCustomDate contains start and end date

Output: Overlapping dates (as List of MyCustomDate)

Each time span includes a start and end point of type LocalDateTime with hours and seconds.

Answer 1

My answer will consider:

• Given (d3, d5) not overlapping => overlap(d1,d3,d5) = 2 as at a given time only two dates will overlap.

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;

class Event {
    LocalDate startDate; // inclusive
    LocalDate endDate; // inclusive

    Event(LocalDate st, LocalDate end) {
        this.startDate = st;
        this.endDate = end;
    }

    // Getters & Setters omitted
}

public class Main {
    public static void main(String[] args) {
        List<Event> events = new ArrayList<Event>();
        events.add(new Event(LocalDate.of(2019,1,1), LocalDate.of(2019,5,1))); // d1
        events.add(new Event(LocalDate.of(2019,3,1), LocalDate.of(2019,6,1))); // d2
        events.add(new Event(LocalDate.of(2019,2,1), LocalDate.of(2019,7,1))); // d3
        events.add(new Event(LocalDate.of(2019,8,1), LocalDate.of(2019,12,1))); // d4
        // d5 do not overlap d3
        events.add(new Event(LocalDate.of(2018,12,1), LocalDate.of(2019,1,31))); // d5

        Integer startDateOverlaps = events.stream().map(Event::getStartDate).mapToInt(date -> overlap(date, events)).max().orElse(0);
        Integer endDateOverlaps = events.stream().map(Event::getEndDate).mapToInt(date -> overlap(date, events)).max().orElse(0);

        System.out.println(Integer.max(startDateOverlaps, endDateOverlaps));
    }

    public static Integer overlap(LocalDate date, List<Event> events) {
        return events.stream().mapToInt(event -> (! (date.isBefore(event.startDate) || date.isAfter(event.endDate))) ? 1 : 0).sum();
    }
}

We sum each overlapping date (even itself as otherwise (d1, d2, d3) would only count (d2, d3) for d1 check) and test each startDate & endDate.