18. Solution: Stream API

SummarizeClients.java

import java.time.LocalDate;
import java.time.Year;
import java.time.ZoneId;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public final class SummarizeClients {
  public static void main(String[] args) {

    List<UdacisearchClient> clients = ClientStore.getClients();
    int numClients = clients.size();
    int totalQuarterlySpend =
        clients
            .stream()
            .mapToInt(UdacisearchClient::getQuarterlyBudget)
            .sum();

    double averageBudget =
        clients
            .stream()
            .mapToDouble(UdacisearchClient::getQuarterlyBudget)
            .average()
            .orElse(0);

    long nextExpiration =
        clients
            .stream()
            .min(Comparator.comparing(UdacisearchClient::getContractEnd))
            .map(UdacisearchClient::getId)
            .orElse(-1L);

    List<ZoneId> representedZoneIds =
        clients
            .stream()
            .flatMap(c -> c.getTimeZones().stream())
            .distinct()  // Or use Collectors.toSet()
            .collect(Collectors.toList());

    Map<Year, Long> contractsPerYear =
        clients
            .stream()
            .collect(Collectors.groupingBy(SummarizeClients::getContractYear, Collectors.counting()));

    System.out.println("Num clients: " + numClients);
    System.out.println("Total quarterly spend: " + totalQuarterlySpend);
    System.out.println("Average budget: " + averageBudget);
    System.out.println("ID of next expiring contract: " + nextExpiration);
    System.out.println("Client time zones: " + representedZoneIds);
    System.out.println("Contracts per year: " + contractsPerYear);
  }

  private static Year getContractYear(UdacisearchClient client) {
    LocalDate contractDate =
        LocalDate.ofInstant(client.getContractStart(), client.getTimeZones().get(0));
    return Year.of(contractDate.getYear());
  }
}