How to Add up the values in a Nested Collection using Streams

Issue

I have the following TicketDTO Object:

public class TicketDTO {
    private LocalDate date;
    private Set<OffenceDTO> offences;
}

And every OffenceDTO has an int field - penalty points.

public class OffenceDTO {
    private int penaltyPoints;
}

I would like to add up the penalty points to a single int value by streaming the Set of Offenses of each Ticket. But only if the ticket's date is between the last two years.

I have collected tickets from the last two years, but now I have a problem in how to go through the offenses and count their points.

This is what I've written so far:

tickets().stream()
    .filter(ticketEntity -> isDateBetween(LocalDate.now(), ticketEntity.getDate()))
    .collect(Collectors.toList());

Solution

I would like to collect the penalty points in a single int value by streaming the set of tickets

It can be done in the following steps:

• Turn the stream of filtered tickets into a stream of OffenceDTO using flatMap();
• Extract penalty points from OffenceDTO with mapToInt(), that will transform a stream of objects into a IntStream;
• Apply sum() to get the total.

int totalPenalty = tickets().stream()
    .filter(ticketEntity -> isDateBetween(LocalDate.now(), ticketEntity.getDate()))
    .flatMap(ticketDTO -> ticketDTO.getOffences().stream())
    .mapToInt(OffenceDTO::getPenaltyPoints)
    .sum();

Answered By - Alexander Ivanchenko