문자열 등급과 타입 안전성

public final class StringGradeTypoBug {
    public static void main(String[] args) {
        DiscountService service = new DiscountService();
        int price = 10_000;

        System.out.println("DIAMOND=" + service.discount("DIAMOND", price));
        System.out.println("DIAMONDD=" + service.discount("DIAMONDD", price));
        System.out.println("gold=" + service.discount("gold", price));
    }

    private static final class DiscountService {
        int discount(String grade, int price) {
            int percent = switch (grade) {
                case "BASIC" -> 10;
                case "GOLD" -> 20;
                case "DIAMOND" -> 30;
                default -> 0;
            };
            return price * percent / 100;
        }
    }
}

DIAMOND=3000
DIAMONDD=0
gold=0

public final class StringGradeConstants {
    public static void main(String[] args) {
        DiscountService service = new DiscountService();
        System.out.println(service.discount(GradeName.GOLD, 10_000));
        try {
            System.out.println(service.discount("SILVER", 10_000));
        } catch (IllegalArgumentException error) {
            System.out.println("rejected=SILVER");
        }
    }

    private static final class GradeName {
        static final String BASIC = "BASIC";
        static final String GOLD = "GOLD";
        static final String DIAMOND = "DIAMOND";
        private GradeName() { }
    }

    private static final class DiscountService {
        int discount(String grade, int price) {
            return switch (grade) {
                case GradeName.BASIC -> price / 10;
                case GradeName.GOLD -> price / 5;
                case GradeName.DIAMOND -> price * 3 / 10;
                default -> throw new IllegalArgumentException("grade=" + grade);
            };
        }
    }
}

2000
rejected=SILVER

import java.util.Locale;

public final class ValidatedGradeText {
    public static void main(String[] args) {
        System.out.println(parse(" gold "));
        try {
            System.out.println(parse("silver"));
        } catch (IllegalArgumentException error) {
            System.out.println("rejected=" + error.getMessage());
        }
    }

    private static String parse(String raw) {
        if (raw == null) throw new IllegalArgumentException("grade=null");
        String normalized = raw.strip().toUpperCase(Locale.ROOT);
        boolean allowed = normalized.equals("BASIC")
                || normalized.equals("GOLD")
                || normalized.equals("DIAMOND");
        if (!allowed) throw new IllegalArgumentException("grade=" + raw);
        return normalized;
    }
}

GOLD
rejected=grade=silver

import java.util.Arrays;
import java.util.Random;

public final class UniqueFeaturedPostIds {
    public static void main(String[] args) {
        FeaturedPostIdGenerator generator = new FeaturedPostIdGenerator(new Random(12));
        System.out.println(Arrays.toString(generator.generate(6)));
    }

    private static final class FeaturedPostIdGenerator {
        private final Random random;

        FeaturedPostIdGenerator(Random random) {
            this.random = random;
        }

        int[] generate(int count) {
            if (count < 1 || count > 45) throw new IllegalArgumentException("count");
            int[] selected = new int[count];
            int size = 0;

            while (size < count) {
                int candidate = random.nextInt(45) + 1;
                boolean duplicate = false;
                for (int index = 0; index < size; index++) {
                    if (selected[index] == candidate) {
                        duplicate = true;
                        break;
                    }
                }
                if (!duplicate) selected[size++] = candidate;
            }

            Arrays.sort(selected);
            return selected;
        }
    }
}

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