groupingBy: การจัดหมวดหมู่องค์ประกอบ

groupingBy คืออะไร

import java.util.*;
import java.util.stream.*;

record Person(String name, String city) {}
List<Person> people = List.of(
    new Person("Alice","NYC"), new Person("Bob","LA"),
    new Person("Carol","NYC"), new Person("Dave","LA"),
    new Person("Eve","Chicago")
);

Map<String, List<Person>> byCity =
    people.stream().collect(Collectors.groupingBy(Person::city));
byCity.forEach((city, ps) -> System.out.println(city + ": " + ps.size()));

ตัวรวบรวมปลายทาง: counting

Map<String, Long> countByCity =
    people.stream().collect(
        Collectors.groupingBy(Person::city, Collectors.counting())
    );
countByCity.forEach((city, n) -> System.out.println(city + " -> " + n));
// NYC -> 2, LA -> 2, Chicago -> 1

ตัวรวบรวมปลายทาง: mapping

Map<String, List<String>> namesByCity =
    people.stream().collect(
        Collectors.groupingBy(
            Person::city,
            Collectors.mapping(Person::name, Collectors.toList())
        )
    );
namesByCity.forEach((city, names) -> System.out.println(city + ": " + names));

ตัวรวบรวมปลายทาง: joining

Map<String, String> joinedByCity =
    people.stream().collect(
        Collectors.groupingBy(
            Person::city,
            Collectors.mapping(Person::name, Collectors.joining(", "))
        )
    );
joinedByCity.forEach((city, s) -> System.out.println(city + ": " + s));
// NYC: Alice, Carol

groupingBy หลายระดับ

record Employee(String name, String dept, String level) {}
List<Employee> emps = List.of(
    new Employee("A","Eng","Junior"), new Employee("B","Eng","Senior"),
    new Employee("C","HR","Junior"), new Employee("D","HR","Senior")
);

Map<String, Map<String, List<Employee>>> grouped =
    emps.stream().collect(
        Collectors.groupingBy(Employee::dept,
            Collectors.groupingBy(Employee::level))
    );
grouped.forEach((dept, levels) -> levels.forEach((level, list) ->
    System.out.println(dept+"/"+level+": "+list.size())));

การควบคุมชนิดของ Map

Map<String, Long> sorted =
    people.stream().collect(
        Collectors.groupingBy(Person::city, TreeMap::new, Collectors.counting())
    );
System.out.println(sorted); // keys in alphabetical order

ตัวรวบรวมปลายทาง: averagingInt

record Product(String category, int price) {}
List<Product> products = List.of(
    new Product("A",10), new Product("A",20),
    new Product("B",15), new Product("B",25)
);

Map<String, Double> avgPrice =
    products.stream().collect(
        Collectors.groupingBy(Product::category,
            Collectors.averagingInt(Product::price))
    );
avgPrice.forEach((cat, avg) -> System.out.printf("%s: %.1f%n", cat, avg));

ตัวรวบรวมปลายทาง: toUnmodifiableList

Map<String, List<String>> immutable =
    people.stream().collect(
        Collectors.groupingBy(Person::city,
            Collectors.mapping(Person::name, Collectors.toUnmodifiableList()))
    );
// Attempting to modify throws UnsupportedOperationException

กรณีใช้งานจริง: สรุปคำสั่งซื้อ

record Order(String status, double amount) {}
List<Order> orders = List.of(
    new Order("PAID", 50.0), new Order("PENDING", 30.0),
    new Order("PAID", 75.0), new Order("CANCELLED", 20.0)
);

Map<String, Double> totalByStatus =
    orders.stream().collect(
        Collectors.groupingBy(Order::status,
            Collectors.summingDouble(Order::amount))
    );
totalByStatus.forEach((s, t) -> System.out.printf("%s: $%.2f%n", s, t));

ข้อควรพิจารณาด้านประสิทธิภาพ

การใช้ร่วมกับ filter และ sorted

Map<String, Long> activeCities =
    people.stream()
          .filter(p -> !p.city().equals("Chicago"))
          .collect(Collectors.groupingBy(Person::city, Collectors.counting()));
System.out.println(activeCities); // {NYC=2, LA=2}

ตรวจสอบความเข้าใจ

สรุป: groupingBy