생성자와 this

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

        System.out.println(member.email.toUpperCase());
    }

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

Exception in thread "main" java.lang.NullPointerException

Member member = new Member();
member.email = email;
member.age = age;

public final class ConstructorMember {
    public static void main(String[] args) {
        Member member = new Member("[email protected]", 40);
        System.out.println(member.describe());
    }

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

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

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

[email protected]=40

public final class MissingThisAssignment {
    public static void main(String[] args) {
        Member member = new Member("[email protected]", 50);
        System.out.println(member.email + "=" + member.age);
    }

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

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

null=0

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

public final class ValidatedConstructor {
    public static void main(String[] args) {
        printCreation("[email protected]", 40);
        printCreation("", 30);
        printCreation("[email protected]", 130);
    }

    private static void printCreation(String email, int age) {
        try {
            Member member = new Member(email, age);
            System.out.println("created=" + member.describe());
        } catch (IllegalArgumentException exception) {
            System.out.println("rejected=" + exception.getMessage());
        }
    }

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

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

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

[email protected]=40
rejected=email
rejected=age

public final class ConstructingMemberRegistry {
    public static void main(String[] args) {
        MemberRegistry registry = new MemberRegistry(2);

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

        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;

        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;
        }

        int age() {
            return age;
        }
    }
}

true
true
false
count=2, total=90