객체의 데이터와 책임

public final class PublicFieldInvariantFailure {
    public static void main(String[] args) {
        Member member = new Member();
        member.email = "[email protected]";
        member.age = 40;

        member.email = null;

        System.out.println(member.describe());
    }

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

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

Exception in thread "main" java.lang.NullPointerException

public final class MemberResponsibility {
    public static void main(String[] args) {
        Member member = new Member("[email protected]", 40);

        member.celebrateBirthday();
        System.out.println(member.describe());
        System.out.println("adult=" + member.isAdult());
    }

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

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

        void celebrateBirthday() {
            if (age >= 120) {
                throw new IllegalStateException("maximum age");
            }
            age++;
        }

        boolean isAdult() {
            return age >= 18;
        }

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

[email protected]=41
adult=true

public final class ObjectOrientedMemberRegistry {
    public static void main(String[] args) {
        MemberRegistry registry = new MemberRegistry(3);

        System.out.println(registry.add("[email protected]", 40));
        System.out.println(registry.add("", 30));
        System.out.println(registry.add("[email protected]", 50));

        registry.printEntries();
        System.out.println("count=" + registry.count());
        System.out.println("total=" + registry.totalAge());
    }

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

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

        boolean add(String email, int age) {
            if (size == members.length || !Member.isValid(email, age)) {
                return false;
            }
            members[size] = new Member(email, age);
            size++;
            return true;
        }

        int count() {
            return size;
        }

        int totalAge() {
            int total = 0;
            for (int index = 0; index < size; index++) {
                total += members[index].age();
            }
            return total;
        }

        void printEntries() {
            for (int index = 0; index < size; index++) {
                System.out.println((index + 1) + ". " + members[index].describe());
            }
        }
    }

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

        Member(String email, int age) {
            if (!isValid(email, age)) {
                throw new IllegalArgumentException("invalid member");
            }
            this.email = email;
            this.age = age;
        }

        static boolean isValid(String email, int age) {
            return email != null && !email.isBlank() && age >= 14 && age <= 120;
        }

        int age() {
            return age;
        }

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

true
false
true
1. [email protected]=40
2. [email protected]=50
count=2
total=90

size = add(members, size, email, age);
int total = total(members, size);
print(members, size);

registry.add(email, age);
int total = registry.totalAge();
registry.printEntries();

registry.add(email, age)
≈ add(registry, email, age)

import java.util.Scanner;

public final class MemberRegistryCollaboration {
    public static void main(String[] args) {
        Scanner scanner = new Scanner("add [email protected] 40\nadd [email protected] 50\nsummary\nquit\n");
        MemberRegistry registry = new MemberRegistry(2);

        while (scanner.hasNextLine()) {
            String[] parts = scanner.nextLine().trim().split("\\s+");
            if (parts[0].equals("quit")) {
                break;
            }
            if (parts[0].equals("summary")) {
                System.out.println(registry.summary());
                continue;
            }
            if (parts.length == 3 && parts[0].equals("add")) {
                boolean saved = registry.add(parts[1], Integer.parseInt(parts[2]));
                System.out.println(saved ? "saved" : "rejected");
            }
        }
    }

    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 || email.isBlank() || age < 14 || age > 120) {
                return false;
            }
            members[size++] = new Member(email, age);
            return true;
        }

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

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

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

saved
saved
count=2, total=90