2. 자바 인터페이스 완전 정복

    abstract class Shape {
        abstract void area();
    }

    class Circle extends Shape {
        double radius;

        Circle(double radius) {
            this.radius = radius;
        }

        @Override
        void area() {
            double result = Math.PI * radius * radius;
            System.out.println("원의 넓이: " + result);
        }
    }

    class Rectangle extends Shape {
        double width, height;

        Rectangle(double width, double height) {
            this.width = width;
            this.height = height;
        }

        @Override
        void area() {
            System.out.println("사각형의 넓이: " + (width * height));
        }
    }

    public class Main {
        public static void main(String[] args) {
            Shape c = new Circle(5);
            Shape r = new Rectangle(3, 4);

            c.area();  // 원의 넓이: 78.539...
            r.area();  // 사각형의 넓이: 12.0
        }
    }
  

    interface Playable {
        void play();
    }

    class Guitar implements Playable {
        @Override
        public void play() {
            System.out.println("기타를 연주합니다!");
        }
    }

    class Piano implements Playable {
        @Override
        public void play() {
            System.out.println("피아노를 연주합니다!");
        }
    }

    public class Main {
        public static void main(String[] args) {
            Playable[] instruments = { new Guitar(), new Piano() };

            for (Playable p : instruments) {
                p.play();
            }
        }
    }
  

    interface Notifiable {
        void send(String message);
    }

    class EmailNotification implements Notifiable {
        @Override
        public void send(String message) {
            System.out.println("[이메일] " + message);
        }
    }

    class SmsNotification implements Notifiable {
        @Override
        public void send(String message) {
            System.out.println("[SMS] " + message);
        }
    }

    public class Main {
        public static void main(String[] args) {
            Notifiable[] channels = {
                new EmailNotification(),
                new SmsNotification()
            };

            for (Notifiable n : channels) {
                n.send("주문이 완료되었습니다!");
            }
        }
    }
  

    abstract class Vehicle {
        abstract void drive();

        void stop() {
            System.out.println("정지합니다.");
        }
    }

    interface Electric {
        void charge();
    }

    class ElectricCar extends Vehicle implements Electric {
        @Override
        void drive() {
            System.out.println("전기차가 조용히 달립니다.");
        }

        @Override
        public void charge() {
            System.out.println("전기차를 충전합니다.");
        }
    }

    public class Main {
        public static void main(String[] args) {
            ElectricCar car = new ElectricCar();
            car.drive();   // 전기차가 조용히 달립니다.
            car.stop();    // 정지합니다.
            car.charge();  // 전기차를 충전합니다.
        }
    }
  

    interface Callable {
        void call(String number);
    }

    interface Browsable {
        void browse(String url);
    }

    interface Photographable {
        void takePhoto();
    }

    class SmartPhone implements Callable, Browsable, Photographable {
        @Override
        public void call(String number) {
            System.out.println(number + "에 전화를 겁니다.");
        }

        @Override
        public void browse(String url) {
            System.out.println(url + "에 접속합니다.");
        }

        @Override
        public void takePhoto() {
            System.out.println("사진을 촬영합니다.");
        }
    }

    public class Main {
        public static void main(String[] args) {
            SmartPhone phone = new SmartPhone();
            phone.call("010-1234-5678");
            phone.browse("www.google.com");
            phone.takePhoto();

            // 인터페이스 타입으로도 사용 가능 (다형성!)
            Callable c = phone;
            c.call("010-9876-5432");
        }
    }
  

    interface Sortable {
        int compareTo(Student other);
    }

    class Student implements Sortable {
        String name;
        int score;

        Student(String name, int score) {
            this.name = name;
            this.score = score;
        }

        @Override
        public int compareTo(Student other) {
            return this.score - other.score;
        }

        void showInfo() {
            System.out.println(name + ": " + score + "점");
        }
    }

    public class Main {
        public static void main(String[] args) {
            Student[] students = {
                new Student("철수", 85),
                new Student("영희", 92),
                new Student("민수", 78)
            };

            // 버블 정렬 (점수 내림차순)
            for (int i = 0; i < students.length - 1; i++) {
                for (int j = 0; j < students.length - 1 - i; j++) {
                    if (students[j].compareTo(students[j + 1]) < 0) {
                        Student temp = students[j];
                        students[j] = students[j + 1];
                        students[j + 1] = temp;
                    }
                }
            }

            for (Student s : students) {
                s.showInfo();
            }
            // 영희: 92점
            // 철수: 85점
            // 민수: 78점
        }
    }