public class App4 {
    public static void main(String[] args) {
        // 실행할 코드

        iPhone iphone = new iPhone();

        iphone.call("010-5556-8614");

        iphone.lock();

        iphone.call("119");

        iphone.unlock();

        iphone.call("112");
        
    }
    
}

abstract class SmartPhone{
    // 전화 걸기
    abstract void call(String number);

    // 잠금 기능
    abstract void lock();

    // 잠금 해체
    abstract void unlock();
}

// Ctrl + .
class iPhone extends SmartPhone{
    Scanner sc = new Scanner(System.in);

    private boolean locked = false;
    private int pin;

    @Override
    void call(String number) {
        if(locked == false){
            System.out.println("calling on iPhone : " + number);
        }
        else{
            System.out.println("Message : Unlock first!");
        }
    }

    @Override
    void lock() {
        if(locked == false){
            System.out.println("Input Password : ");
            pin = sc.nextInt();
            locked = true;
        }
        else{
            System.out.println("Phone is already locked!");
        }
    }

    @Override
    void unlock() {
        if(locked == true){
            System.out.println("PIN: ");
            int input = sc.nextInt();
            if(input == pin){
                System.out.println("Correct PIN!");
                locked = false;
            }
            else{
                System.out.println("Wrong PIN! Try Again!");
            }
        }
        else{
            System.out.println("Phone is already unlocked!");
        }
    }
}

[edit]

3.2. 예제2 ¶

public class App5 {
    public static void main(String[] args) {
        AirAttacker atk = new AirAttacker();

        atk.attack("Player");

        atk.fly(10, 5, 6);

    }
}

interface Flyable{
    void fly(int x, int y, int z);
}

interface Moveable{
    void move(int x, int y);
}

interface Attackable{
    void attack(String target);
}

abstract class Unit{
    int hp;
    int mp;
    int exp;

    Unit(int hp, int mp, int exp){
        this.hp = hp;
        this.mp = mp;
        this.exp = exp;
    }

    abstract void attacked(int damage);
    abstract void defeated();
}

interface FlyAttacker extends Flyable, Attackable{
    abstract void airToground();
}

class AirAttacker implements Flyable, Attackable{

    @Override
    public void attack(String target) {
        System.out.println("Attack on : " + target);
        
    }

    @Override
    public void fly(int x, int y, int z) {
        System.out.println("Move to : " + x + y + z);
        
    }
}

class Dragon extends Unit implements FlyAttacker{

    Dragon(int hp, int mp, int exp) {
        super(hp, mp, exp);
        //TODO Auto-generated constructor stub
    }

    @Override
    public void attack(String target) {
        // TODO Auto-generated method stub

    }        

    @Override
    public void fly(int x, int y, int z) {
        // TODO Auto-generated method stub
        
    }

    @Override
    void attacked(int damage) {
        // TODO Auto-generated method stub
        
    }

    @Override
    void defeated() {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void airToground() {
        // TODO Auto-generated method stub
        
    }

}

[edit]