캡슐화된 회원 저장소

private static final class MemberClass {
    private final String email;
    private final int age;

    MemberClass(String email, int age) {
        this.email = email;
        this.age = age;
    }

    String email() { return email; }
    int age() { return age; }
}

private record Member(String email, int age) {}

public final class ExposedArrayBug {
    public static void main(String[] args) {
        MemberRegistry registry = new MemberRegistry();
        registry.add("[email protected]", 40);

        Member[] leaked = registry.members();
        leaked[0] = null;

        System.out.println(registry.firstDescription());
    }

    private static final class MemberRegistry {
        private final Member[] members = new Member[2];
        private int size;

        void add(String email, int age) {
            members[size++] = new Member(email, age);
        }

        Member[] members() {
            return members;
        }

        String firstDescription() {
            return members[0].describe();
        }
    }

    private record Member(String email, int age) {
        String describe() {
            return email + "=" + age;
        }
    }
}

Exception in thread "main" java.lang.NullPointerException

public final class EncapsulatedMemberRegistry {
    public static void main(String[] args) {
        MemberRegistry registry = new MemberRegistry(3);
        registry.add("[email protected]", 40);
        registry.add("[email protected]", 30);
        registry.add("[email protected]", 50);

        System.out.println(registry.removeAt(1));
        System.out.println(registry.describeAt(0));
        System.out.println(registry.describeAt(1));
        System.out.println(registry.summary());
    }

    private static final class MemberRegistry {
        private final Member[] members;
        private int size;

        MemberRegistry(int capacity) {
            if (capacity <= 0) throw new IllegalArgumentException("capacity");
            members = new Member[capacity];
        }

        boolean add(String email, int age) {
            if (size == members.length) return false;
            try {
                members[size] = new Member(email, age);
                size++;
                return true;
            } catch (IllegalArgumentException exception) {
                return false;
            }
        }

        boolean removeAt(int target) {
            if (target < 0 || target >= size) return false;
            for (int index = target; index < size - 1; index++) {
                members[index] = members[index + 1];
            }
            members[size - 1] = null;
            size--;
            return true;
        }

        String describeAt(int index) {
            if (index < 0 || index >= size) throw new IndexOutOfBoundsException(index);
            return members[index].describe();
        }

        String summary() {
            int total = 0;
            for (int index = 0; index < size; index++) {
                total += members[index].age();
            }
            double average = size == 0 ? 0.0 : (double) total / size;
            return "count=" + size + ", total=" + total + ", average=" + average;
        }
    }

    private record Member(String email, int age) {
        Member {
            if (email == null || email.isBlank() || age < 14 || age > 120) {
                throw new IllegalArgumentException("invalid member");
            }
        }

        String describe() {
            return email + "=" + age;
        }
    }
}

true
[email protected]=40
[email protected]=50
count=2, total=90, average=45.0

import java.util.Scanner;

public final class EncapsulatedMemberRegistryCli {
    public static void main(String[] args) {
        String input = String.join("\n",
                "add [email protected] 40",
                "add bad 0",
                "add [email protected] 50",
                "list",
                "remove 1",
                "summary",
                "quit") + "\n";
        Scanner scanner = new Scanner(input);
        MemberRegistry registry = new MemberRegistry(3);

        while (scanner.hasNextLine()) {
            String line = scanner.nextLine().trim();
            if (line.equals("quit")) break;
            handle(line, registry);
        }
        System.out.println("bye");
    }

    private static void handle(String line, MemberRegistry registry) {
        String[] parts = line.split("\\s+");
        try {
            switch (parts[0]) {
                case "add" -> {
                    if (parts.length != 3) {
                        System.out.println("usage");
                        return;
                    }
                    boolean saved = registry.add(parts[1], Integer.parseInt(parts[2]));
                    System.out.println(saved ? "saved" : "rejected");
                }
                case "remove" -> {
                    if (parts.length != 2) {
                        System.out.println("usage");
                        return;
                    }
                    int oneBased = Integer.parseInt(parts[1]);
                    System.out.println(registry.removeAt(oneBased - 1) ? "removed" : "not-found");
                }
                case "list" -> System.out.print(registry.listText());
                case "summary" -> System.out.println(registry.summary());
                default -> System.out.println("unknown");
            }
        } catch (NumberFormatException exception) {
            System.out.println("number-error");
        }
    }

    private static final class MemberRegistry {
        private final Member[] members;
        private int size;

        MemberRegistry(int capacity) {
            members = new Member[capacity];
        }

        boolean add(String email, int age) {
            if (size == members.length) return false;
            try {
                members[size] = new Member(email, age);
                size++;
                return true;
            } catch (IllegalArgumentException exception) {
                return false;
            }
        }

        boolean removeAt(int target) {
            if (target < 0 || target >= size) return false;
            for (int index = target; index < size - 1; index++) {
                members[index] = members[index + 1];
            }
            members[--size] = null;
            return true;
        }

        String listText() {
            if (size == 0) return "no members\n";
            StringBuilder result = new StringBuilder();
            for (int index = 0; index < size; index++) {
                result.append(index + 1).append(". ").append(members[index].describe()).append('\n');
            }
            return result.toString();
        }

        String summary() {
            int total = 0;
            for (int index = 0; index < size; index++) {
                total += members[index].age();
            }
            return "count=" + size + ", total=" + total;
        }
    }

    private record Member(String email, int age) {
        Member {
            if (email == null || email.isBlank() || age < 14 || age > 120) {
                throw new IllegalArgumentException("invalid member");
            }
        }

        String describe() {
            return email + "=" + age;
        }
    }
}

saved
rejected
saved
1. [email protected]=40
2. [email protected]=50
removed
count=1, total=50
bye