공유 참조와 소유권

public final class SharedEntrySideEffect {
    public static void main(String[] args) {
        MutableEntry entry = new MutableEntry("object", 40);
        DailyReport report = new DailyReport(entry);
        PreviewService preview = new PreviewService(entry);

        preview.applyPreviewBoost(5);

        System.out.println("stored=" + report.totalViewCount());
        System.out.println("preview=" + preview.previewViewCount());
    }

    private static final class MutableEntry {
        private final String title;
        private int viewCount;

        MutableEntry(String title, int viewCount) {
            this.title = title;
            this.viewCount = viewCount;
        }

        void addViewCount(int extra) { viewCount += extra; }
        int viewCount() { return viewCount; }
    }

    private record DailyReport(MutableEntry entry) {
        int totalViewCount() { return entry.viewCount(); }
    }

    private record PreviewService(MutableEntry entry) {
        void applyPreviewBoost(int extra) { entry.addViewCount(extra); }
        int previewViewCount() { return entry.viewCount(); }
    }
}

stored=45
preview=45

public final class SeparatedMutableEntries {
    public static void main(String[] args) {
        MutableEntry stored = new MutableEntry("object", 40);
        MutableEntry previewEntry = stored.copy();

        DailyReport report = new DailyReport(stored);
        PreviewService preview = new PreviewService(previewEntry);
        preview.applyPreviewBoost(5);

        System.out.println("stored=" + report.totalViewCount());
        System.out.println("preview=" + preview.previewViewCount());
    }

    private static final class MutableEntry {
        private final String title;
        private int viewCount;

        MutableEntry(String title, int viewCount) {
            this.title = title;
            this.viewCount = viewCount;
        }

        MutableEntry copy() { return new MutableEntry(title, viewCount); }
        void addViewCount(int extra) { viewCount += extra; }
        int viewCount() { return viewCount; }
    }

    private record DailyReport(MutableEntry entry) {
        int totalViewCount() { return entry.viewCount(); }
    }

    private record PreviewService(MutableEntry entry) {
        void applyPreviewBoost(int extra) { entry.addViewCount(extra); }
        int previewViewCount() { return entry.viewCount(); }
    }
}

stored=40
preview=45

public final class OwnedBoard {
    public static void main(String[] args) {
        Board board = new Board();
        long id = board.add("references", 40);
        board.reviseViewCount(id, 55);

        EntryView[] snapshot = board.snapshot();
        for (EntryView entry : snapshot) System.out.println(entry);
        System.out.println("total=" + board.totalViewCount());
    }

    private record EntryView(long id, String title, int viewCount) { }

    private static final class MutableEntry {
        private final long id;
        private final String title;
        private int viewCount;

        MutableEntry(long id, String title, int viewCount) {
            this.id = id;
            this.title = title;
            this.viewCount = viewCount;
        }
    }

    private static final class Board {
        private final MutableEntry[] entries = new MutableEntry[10];
        private int size;
        private long nextId = 1;

        long add(String title, int viewCount) {
            validateViewCount(viewCount);
            long id = nextId++;
            if (size == entries.length) throw new IllegalStateException("full");
            entries[size++] = new MutableEntry(id, title, viewCount);
            return id;
        }

        void reviseViewCount(long id, int viewCount) {
            validateViewCount(viewCount);
            for (int index = 0; index < size; index++) {
                MutableEntry entry = entries[index];
                if (entry.id == id) {
                    entry.viewCount = viewCount;
                    return;
                }
            }
            throw new IllegalArgumentException("id=" + id);
        }

        EntryView[] snapshot() {
            EntryView[] result = new EntryView[size];
            for (int index = 0; index < size; index++) {
                MutableEntry entry = entries[index];
                result[index] = new EntryView(entry.id, entry.title, entry.viewCount);
            }
            return result;
        }

        int totalViewCount() {
            int total = 0;
            for (int index = 0; index < size; index++) {
                total += entries[index].viewCount;
            }
            return total;
        }

        private static void validateViewCount(int viewCount) {
            if (viewCount <= 0) throw new IllegalArgumentException("viewCount");
        }
    }
}

EntryView[id=1, title=references, viewCount=55]
total=55

long totalViewCountLong() {
    long total = 0;
    for (int index = 0; index < size; index++) {
        total += entries[index].viewCount;
    }
    return total;
}