import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
/**
 * 單例模式：線程安全的數(shù)據庫連接池
 * 使用雙重檢查鎖定（DCL）避免多線程問題
 */
public class DatabaseConnectionPool {
    // volatile確保可見性，防止指令重排
    private static volatile DatabaseConnectionPool instance;
    private Connection connection;
    private DatabaseConnectionPool() throws SQLException {
        // 初始化數(shù)據庫連接（實際生產中會使用連接池框架如HikariCP）
        this.connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "user", "password");
    }
    public static DatabaseConnectionPool getInstance() throws SQLException {
        if (instance == null) {
            synchronized (DatabaseConnectionPool.class) {
                if (instance == null) {
                    instance = new DatabaseConnectionPool();
                }
            }
        }
        return instance;
    }
    public Connection getConnection() {
        return connection;
    }
    // 業(yè)務使用示例
    public static void main(String[] args) {
        try {
            DatabaseConnectionPool pool = DatabaseConnectionPool.getInstance();
            Connection conn = pool.getConnection();
            System.out.println("Database connection established: " + conn);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

/**
 * 工廠方法模式：支付方式工廠
 * 業(yè)務場景：電商系統(tǒng)支持微信、支付寶、信用卡支付
 */
// 抽象產品：支付接口
interface Payment {
    void pay(int amount);
}
// 具體產品：微信支付
class WeChatPay implements Payment {
    @Override
    public void pay(int amount) {
        System.out.println("WeChat Pay: " + amount + "元");
    }
}
// 具體產品：支付寶
class Alipay implements Payment {
    @Override
    public void pay(int amount) {
        System.out.println("Alipay: " + amount + "元");
    }
}
// 工廠：根據類型創(chuàng)建支付對象
class PaymentFactory {
    public Payment createPayment(String type) {
        switch (type.toLowerCase()) {
            case "wechat":
                return new WeChatPay();
            case "alipay":
                return new Alipay();
            default:
                throw new IllegalArgumentException("Unsupported payment type: " + type);
        }
    }
}
// 業(yè)務調用（無需修改代碼即可新增支付方式）
public class PaymentService {
    private PaymentFactory factory = new PaymentFactory();
    public void processPayment(String paymentType, int amount) {
        Payment payment = factory.createPayment(paymentType);
        payment.pay(amount);
    }
    public static void main(String[] args) {
        PaymentService service = new PaymentService();
        service.processPayment("wechat", 100); // 輸出：WeChat Pay: 100元
        service.processPayment("alipay", 200);  // 輸出：Alipay: 200元
    }
}

import java.util.ArrayList;
import java.util.List;
/**
 * 觀察者模式：訂單狀態(tài)通知
 * 業(yè)務場景：用戶下單后，短信、郵件、APP推送同時通知
 */
// 主題（被觀察者）
interface OrderSubject {
    void registerObserver(Observer observer);
    void removeObserver(Observer observer);
    void notifyObservers();
}
// 觀察者接口
interface Observer {
    void update(String orderStatus);
}
// 訂單實體（被觀察者）
class Order implements OrderSubject {
    private List<Observer> observers = new ArrayList<>();
    private String status;
    public void setStatus(String status) {
        this.status = status;
        notifyObservers(); // 狀態(tài)變更時通知所有觀察者
    }
    @Override
    public void registerObserver(Observer observer) {
        observers.add(observer);
    }
    @Override
    public void removeObserver(Observer observer) {
        observers.remove(observer);
    }
    @Override
    public void notifyObservers() {
        for (Observer observer : observers) {
            observer.update(status);
        }
    }
}
// 具體觀察者：短信通知
class SmsObserver implements Observer {
    @Override
    public void update(String status) {
        System.out.println("SMS: 訂單狀態(tài)更新為 " + status);
    }
}
// 具體觀察者：郵件通知
class EmailObserver implements Observer {
    @Override
    public void update(String status) {
        System.out.println("Email: 訂單狀態(tài)更新為 " + status);
    }
}
// 業(yè)務使用
public class OrderNotificationSystem {
    public static void main(String[] args) {
        Order order = new Order();
        // 注冊觀察者
        order.registerObserver(new SmsObserver());
        order.registerObserver(new EmailObserver());
        // 更新訂單狀態(tài)（自動觸發(fā)通知）
        order.setStatus("Shipped"); // 輸出：SMS: 訂單狀態(tài)更新為 Shipped
                                 //       Email: 訂單狀態(tài)更新為 Shipped
    }
}

/**
 * 策略模式：促銷策略
 * 業(yè)務場景：電商大促，支持滿減、折扣、無優(yōu)惠三種策略
 */
// 策略接口
interface PromotionStrategy {
    double calculateDiscount(double originalPrice);
}
// 具體策略：滿減（滿100減20）
class FullReduction implements PromotionStrategy {
    @Override
    public double calculateDiscount(double originalPrice) {
        return originalPrice > 100 ? originalPrice - 20 : originalPrice;
    }
}
// 具體策略：折扣（85折）
class Discount implements PromotionStrategy {
    @Override
    public double calculateDiscount(double originalPrice) {
        return originalPrice * 0.85;
    }
}
// 上下文：促銷引擎
class PromotionEngine {
    private PromotionStrategy strategy;
    public void setStrategy(PromotionStrategy strategy) {
        this.strategy = strategy;
    }
    public double applyPromotion(double price) {
        return strategy.calculateDiscount(price);
    }
}
// 業(yè)務使用
public class PromotionDemo {
    public static void main(String[] args) {
        PromotionEngine engine = new PromotionEngine();
        // 設置滿減策略
        engine.setStrategy(new FullReduction());
        System.out.println("FullReduction: " + engine.applyPromotion(120)); // 輸出: 100.0
        // 切換為折扣策略
        engine.setStrategy(new Discount());
        System.out.println("Discount: " + engine.applyPromotion(120)); // 輸出: 102.0
    }
}

/**
 * 適配器模式：支付SDK適配
 * 業(yè)務場景：接入新支付公司（如PayPal）的SDK，但原有系統(tǒng)使用老接口
 */
// 目標接口（系統(tǒng)期望的）
interface OldPaymentGateway {
    void processPayment(double amount);
}
// 適配器：將PayPalSDK適配到OldPaymentGateway
class PayPalAdapter implements OldPaymentGateway {
    private PayPalSDK payPalSDK;
    public PayPalAdapter(PayPalSDK payPalSDK) {
        this.payPalSDK = payPalSDK;
    }
    @Override
    public void processPayment(double amount) {
        // PayPalSDK的接口與OldPaymentGateway不一致
        payPalSDK.makePayment(amount);
    }
}
// 第三方SDK（無法修改）
class PayPalSDK {
    public void makePayment(double amount) {
        System.out.println("PayPal SDK: Processing payment of " + amount);
    }
}
// 業(yè)務使用（無需修改原有代碼）
public class PaymentAdapterDemo {
    public static void main(String[] args) {
        OldPaymentGateway gateway = new PayPalAdapter(new PayPalSDK());
        gateway.processPayment(50.0); // 輸出：PayPal SDK: Processing payment of 50.0
    }
}