커스텀 Collector 만들기

Java code

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

public class CustomCollector {
    // 문자열을 역순으로 연결하는 Collector
    static Collector<String, ?, String> reverseJoining(String delimiter) {
        return Collector.of(
            // supplier: 컨테이너 생성
            () -> new ArrayDeque<String>(),
            // accumulator: 원소 추가
            (deque, item) -> deque.addFirst(item),
            // combiner: 병렬 시 결합
            (left, right) -> { right.addAll(left); return right; },
            // finisher: 최종 변환
            deque -> String.join(delimiter, deque),
            // characteristics
            Collector.Characteristics.IDENTITY_FINISH
        );
    }

    // Top N 수집기
    static <T extends Comparable<T>> Collector<T, ?, List<T>> topN(int n) {
        return Collector.of(
            () -> new PriorityQueue<T>(n),
            (pq, item) -> {
                pq.add(item);
                if (pq.size() > n) pq.poll();
            },
            (left, right) -> {
                left.addAll(right);
                while (left.size() > n) left.poll();
                return left;
            },
            pq -> {
                List<T> result = new ArrayList<>(pq);
                Collections.sort(result, Comparator.reverseOrder());
                return result;
            }
        );
    }

    public static void main(String[] args) {
        String result = List.of("A", "B", "C").stream()
            .collect(reverseJoining(" -> "));
        System.out.println(result); // C -> B -> A

        List<Integer> top3 = java.util.stream.IntStream.range(1, 100)
            .boxed().collect(topN(3));
        System.out.println(top3); // [99, 98, 97]
    }
}