public class BankAccount {
           private int balance;
 
           public void deposit(int amount) {
               if (amount > 0) balance += amount;
           }
 
           public void withdraw(int amount) {
               if (amount <= balance) balance -= amount;
           }
 
           public int getBalance() { return balance; }
       }           

       public class Animal {
           public void eat() { System.out.println("밥을 먹는다."); }
       }
 
       public class Dog extends Animal {
           public void fetch() { System.out.println("공을 물어온다."); }
       }

       public interface Payment {
           void pay(int amount);
       }
 
       public class CardPayment implements Payment {
           @Override
           public void pay(int amount) { System.out.println("카드로 " + amount + "원 결제"); }
       }
 
       public class KakaoPay implements Payment {
           @Override
           public void pay(int amount) { System.out.println("카카오페이로 " + amount + "원 결제"); }
       }

       public interface Payment {
           void pay(int amount);
       }
 
       public class CardPayment implements Payment {
           public void pay(int amount) { System.out.println("카드로 " + amount + "원 결제"); }
       }
 
       public class KakaoPay implements Payment {
           public void pay(int amount) { System.out.println("카카오페이로 " + amount + "원 결제"); }
       }

    // 약속
    public interface PostRepository {
        Post save(Post post);
    }
    
    // 구현
    public class MemoryPostRepository implements PostRepository {
        private final Map<Long, Post> store = new HashMap<>();
    
        @Override
        public Post save(Post post) {
            store.put(post.getId(), post);
            return post;
        }
    }
    
    // 사용: Service는 인터페이스만 앎
    public class PostService {
        private final