Streamサンプル improved

import static java.lang.System.out;// shorter & simplified
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {

    static record Person(String name, int age) {// immutable
        public String getName() {
            return name;
        }

        public int getAge() {
            return age;
        }
    }

    public static List<Person> getPeople() {// better naming
        return List.of
        (
            new Person("Alice", 25),
            new Person("Bob", 30),
            new Person("Charlie", 35),
            new Person("Dave", 35)
        );// immutable & more concise
    }

    public static void main(String[] args) {
        var personList = getPeople();

        // nameを取り出し
        List<String> upperCaseNames = personList.stream()
                .map(person -> person.getName().toUpperCase())
                .toList();// shortened
                
        out.println("* map result1");
        out.println(String.join(",", upperCaseNames));

        // nameを取り出し
        List<String> names = personList.stream()
                .map(Person::getName)
                .toList();
                
        out.println("* map result2");
        out.println(String.join(",", names));

        // Filter処理
        List<Person> filteredList = personList.stream()
                .filter(person -> person.getAge() >= 30)
                .toList();

        out.println("* filter result");
        filteredList.forEach(out::println);// simplified

        // Group by 処理
        Map<Integer, Long> ageCount = personList.stream()
                .collect(Collectors.groupingBy(Person::getAge, Collectors.counting()));
                
                
        out.println("* group-by result");
        ageCount.keySet().stream().forEach(key -> {
            out.printf("%s = %s\n", key, ageCount.get(key));
        });
    }
}