class UserService {
    public void registerUser() {
        // 회원가입 로직
    }

    public void sendWelcomeEmail() {
        // 이메일 전송 로직
    }
}

class UserService {
    private EmailService emailService;

    public void registerUser() {
        // 회원가입 로직
        emailService.sendWelcomeEmail();
    }
}

class EmailService {
    public void sendWelcomeEmail() {
        // 이메일 전송
    }
}

class DiscountService {
    public int calculateDiscount(String grade, int price) {
        if (grade.equals("VIP")) return price - 1000;
        else return price - 500;
    }
}

interface DiscountPolicy {
    int discount(int price);
}

class VIPDiscount implements DiscountPolicy {
    public int discount(int price) {
        return price - 1000;
    }
}

class BasicDiscount implements DiscountPolicy {
    public int discount(int price) {
        return price - 500;
    }
}

class DiscountService {
    public int calculate(DiscountPolicy policy, int price) {
        return policy.discount(price);
    }
}

class Rectangle {
    protected int width, height;

    public void setWidth(int w) { width = w; }
    public void setHeight(int h) { height = h; }

    public int getArea() { return width * height; }
}

class Square extends Rectangle {
    @Override
    public void setWidth(int w) {
        width = height = w;
    }

    @Override
    public void setHeight(int h) {
        width = height = h;
    }
}

interface Shape {
    int getArea();
}

class Rectangle implements Shape {
    private int width, height;

    Rectangle(int w, int h) {
        this.width = w;
        this.height = h;
    }

    public int getArea() {
        return width * height;
    }
}

class Square implements Shape {
    private int side;

    Square(int s) {
        this.side = s;
    }

    public int getArea() {
        return side * side;
    }
}

interface Animal {
    void fly();
    void walk();
}

class Dog implements Animal {
    public void fly() {
        // 아무 동작 안함
    }

    public void walk() {
        // 걷기
    }
}

interface Walkable {
    void walk();
}

interface Flyable {
    void fly();
}

class Dog implements Walkable {
    public void walk() {
        // 걷기
    }
}

class MySQLRepository {
    public void save() {
        // DB 저장
    }
}

class UserService {
    private MySQLRepository repo = new MySQLRepository();

    public void register() {
        repo.save();
    }
}

interface Repository {
    void save();
}

class MySQLRepository implements Repository {
    public void save() {
        // DB 저장
    }
}

class UserService {
    private final Repository repo;

    public UserService(Repository repo) {
        this.repo = repo;
    }

    public void register() {
        repo.save();
    }
}