Java Streams - Collectors collectingAndThen (Collector downstream, Function finisher)








Collectors collectingAndThen(Collector<T,A,R> downstream, Function<R,RR> finisher) adapts a Collector to perform an additional finishing transformation.

Syntax

collectingAndThen has the following syntax.

public static <T,A,R,RR> Collector<T,A,RR> collectingAndThen(Collector<T,A,R> downstream,      Function<R,RR> finisher)

Example

The following example shows how to use collectingAndThen.

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.stream.Collectors;
/*w  ww .  j a  v  a 2  s .  c  o  m*/
public class Main {
  public static void main(String[] args) {
    Map<Month, String> dobCalendar = Employee
        .persons()
        .stream()
        .collect(
            Collectors.collectingAndThen(
                Collectors.groupingBy(
                    p -> p.getDob().getMonth(),
                    Collectors.mapping(Employee::getName,
                        Collectors.joining(" "))), result -> {
                for (Month m : Month.values()) {
                  result.putIfAbsent(m, "None");
                }
                return Collections.unmodifiableMap(new TreeMap<>(result));
              }));

    dobCalendar.entrySet().forEach(System.out::println);
  }
}

enum Gender {
  MALE, FEMALE
}

class Employee {
  private long id;
  private String name;
  private Gender gender;
  private LocalDate dob;
  private double income;

  public Employee(long id, String name, Gender gender, LocalDate dob,
      double income) {
    this.id = id;
    this.name = name;
    this.gender = gender;
    this.dob = dob;
    this.income = income;
  }

  public String getName() {
    return name;
  }

  public Gender getGender() {
    return gender;
  }

  public LocalDate getDob() {
    return dob;
  }

  public static List<Employee> persons() {
    Employee ken = new Employee(1, "Jame", Gender.MALE, LocalDate.of(1970,
        Month.MAY, 4), 6123.0);
    Employee jeff = new Employee(2, "Jeff", Gender.MALE, LocalDate.of(1971,
        Month.JULY, 5), 7112.0);
    Employee donna = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1972,
        Month.JULY, 9), 8712.0);
    Employee chris = new Employee(4, "Jodd", Gender.MALE, LocalDate.of(1973,
        Month.DECEMBER, 6), 1823.0);
    Employee laynie = new Employee(5, "Janey", Gender.FEMALE, LocalDate.of(
        1974, Month.DECEMBER, 3), 1234.0);
    Employee lee = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1975,
        Month.MAY, 8), 2412.0);

    // Create a list of persons
    List<Employee> persons = Arrays
        .asList(ken, jeff, donna, chris, laynie, lee);

    return persons;
  }
}

The code above generates the following result.