개방-폐쇄 원칙과 전략 교체

public final class UnconfiguredStrategyFailure {
    public static void main(String[] args) {
        ExportService service = new ExportService();
        service.export("가입 인사", 40);
    }

    private interface Exporter {
        void export(String title, int viewCount);
    }

    private static final class ExportService {
        private Exporter exporter;

        void export(String title, int viewCount) {
            exporter.export(title, viewCount);
        }
    }
}

Exception in thread "main" java.lang.NullPointerException

public final class RoleBasedExporter {
    public static void main(String[] args) {
        ExportService textService = new ExportService(new TextExporter());
        ExportService csvService = new ExportService(new CsvExporter());

        textService.export("가입 인사", 40);
        csvService.export("로그인 구현", 50);
    }

    private interface Exporter {
        String format(String title, int viewCount);
    }

    private static final class TextExporter implements Exporter {
        @Override
        public String format(String title, int viewCount) {
            return title + "=" + viewCount;
        }
    }

    private static final class CsvExporter implements Exporter {
        @Override
        public String format(String title, int viewCount) {
            return title + "," + viewCount;
        }
    }

    private static final class ExportService {
        private final Exporter exporter;

        ExportService(Exporter exporter) {
            if (exporter == null) throw new IllegalArgumentException("exporter");
            this.exporter = exporter;
        }

        void export(String title, int viewCount) {
            System.out.println("export:start");
            System.out.println(exporter.format(title, viewCount));
            System.out.println("export:end");
        }
    }
}

export:start
가입 인사=40
export:end
export:start
로그인 구현,50
export:end

public final class OpenClosedExportService {
    public static void main(String[] args) {
        ExportService service = new ExportService(new JsonExporter());
        System.out.println(service.export(new Post("polymorphism", 60)));
    }

    private record Post(String title, int viewCount) {
        Post {
            if (title == null || title.isBlank()) throw new IllegalArgumentException("title");
            if (viewCount <= 0) throw new IllegalArgumentException("viewCount");
        }
    }

    private interface Exporter {
        String format(Post entry);
    }

    private static final class JsonExporter implements Exporter {
        @Override
        public String format(Post entry) {
            return "{\"title\":\"" + entry.title() + "\",\"viewCount\":" + entry.viewCount() + "}";
        }
    }

    private static final class ExportService {
        private final Exporter exporter;

        ExportService(Exporter exporter) {
            this.exporter = exporter;
        }

        String export(Post entry) {
            return exporter.format(entry);
        }
    }
}

{"title":"polymorphism","viewCount":60}

public final class EscapedTitleFormat {
    public static void main(String[] args) {
        String title = "가입 \"인사\", 다음";
        System.out.println("{\"title\":" + jsonString(title) + ",\"viewCount\":40}");
        System.out.println(csvField(title) + ",40");
    }

    private static String jsonString(String value) {
        StringBuilder result = new StringBuilder("\"");
        for (int index = 0; index < value.length(); index++) {
            char character = value.charAt(index);
            if (character == '"') result.append("\\\"");
            else if (character == '\\') result.append("\\\\");
            else if (character < 0x20) {
                result.append(String.format("\\u%04x", (int) character));
            } else result.append(character);
        }
        return result.append('"').toString();
    }

    private static String csvField(String value) {
        return "\"" + value.replace("\"", "\"\"") + "\"";
    }
}

{"title":"가입 \"인사\", 다음","viewCount":40}
"가입 ""인사"", 다음",40

public final class BoardStrategy {
    public static void main(String[] args) {
        Board board = new Board();
        board.add("가입 인사", 40);
        board.add("로그인 구현", 50);

        ExportService text = new ExportService(new TextExporter());
        ExportService csv = new ExportService(new CsvExporter());
        System.out.println(text.export(board.entries()));
        System.out.println(csv.export(board.entries()));
    }

    private record Post(String title, int viewCount) { }

    private static final class Board {
        private final Post[] entries = new Post[10];
        private int size;

        void add(String title, int viewCount) {
            if (title == null || title.isBlank()) throw new IllegalArgumentException("title");
            if (viewCount <= 0) throw new IllegalArgumentException("viewCount");
            if (size == entries.length) throw new IllegalStateException("full");
            entries[size++] = new Post(title, viewCount);
        }

        Post[] entries() {
            Post[] copy = new Post[size];
            for (int index = 0; index < size; index++) {
                copy[index] = entries[index];
            }
            return copy;
        }
    }

    private interface Exporter {
        String format(Post[] entries);
    }

    private static final class TextExporter implements Exporter {
        @Override
        public String format(Post[] entries) {
            String result = "";
            for (int index = 0; index < entries.length; index++) {
                if (index > 0) result += " | ";
                result += entries[index].title() + "=" + entries[index].viewCount();
            }
            return result;
        }
    }

    private static final class CsvExporter implements Exporter {
        @Override
        public String format(Post[] entries) {
            String result = "title,viewCount";
            for (Post entry : entries) {
                result += "\n" + entry.title() + "," + entry.viewCount();
            }
            return result;
        }
    }

    private static final class ExportService {
        private final Exporter exporter;

        ExportService(Exporter exporter) {
            this.exporter = exporter;
        }

        String export(Post[] entries) {
            return exporter.format(entries);
        }
    }
}

가입 인사=40 | 로그인 구현=50
title,viewCount
가입 인사,40
로그인 구현,50

public final class PublicationNotifierStrategy {
    public static void main(String[] args) {
        PublicationService console = new PublicationService(new ConsoleNotifier());
        PublicationService silent = new PublicationService(new SilentNotifier());

        console.publish("다형성 질문");
        silent.publish("상속 질문");
    }

    private interface PublicationNotifier {
        void send(String message);
    }

    private static final class ConsoleNotifier implements PublicationNotifier {
        @Override
        public void send(String message) {
            System.out.println("console=" + message);
        }
    }

    private static final class SilentNotifier implements PublicationNotifier {
        @Override
        public void send(String message) {
            System.out.println("silent");
        }
    }

    private static final class PublicationService {
        private final PublicationNotifier notifier;

        PublicationService(PublicationNotifier notifier) {
            this.notifier = notifier;
        }

        void publish(String title) {
            notifier.send(title + " published");
        }
    }
}

console=다형성 질문 published
silent