设计模式简介

public class Singleton {
    private static volatile Singleton instance;

    private Singleton() {
        // 防止反射
        if (instance != null) {
            throw new RuntimeException("Use getInstance() method to create");
        }
    }

    public static Singleton getInstance() {
        if (instance == null) { // 第一次检查
            synchronized (Singleton.class) {
                if (instance == null) { // 第二次检查
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

// 产品接口
public interface Product {
    void use();
}

// 具体产品类
public class ConcreteProductA implements Product {
    public void use() {
        System.out.println("使用具体产品 A");
    }
}

// 工厂接口
public interface Factory {
    Product createProduct();
}

// 具体工厂类
public class ConcreteFactoryA implements Factory {
    public Product createProduct() {
        return new ConcreteProductA();
    }
}

// 客户端代码
public class Client {
    public static void main(String[] args) {
        Factory factory = new ConcreteFactoryA();
        Product product = factory.createProduct();
        product.use();
    }
}

import java.util.ArrayList;
import java.util.List;

// 观察者接口
public interface Observer {
    void update(String message);
}

// 主题接口
public interface Subject {
    void registerObserver(Observer o);
    void removeObserver(Observer o);
    void notifyObservers();
}

// 具体主题类
public class ConcreteSubject implements Subject {
    private List<Observer> observers = new ArrayList<>();
    private String state;

    public void setState(String state) {
        this.state = state;
        notifyObservers();
    }

    public String getState() {
        return state;
    }

    public void registerObserver(Observer o) {
        observers.add(o);
    }

    public void removeObserver(Observer o) {
        observers.remove(o);
    }

    public void notifyObservers() {
        for (Observer o : observers) {
            o.update(state);
        }
    }
}

// 具体观察者类
public class ConcreteObserver implements Observer {
    private String name;

    public ConcreteObserver(String name){
        this.name = name;
    }

    public void update(String message){
        System.out.println(name + " 接收到更新消息: " + message);
    }
}

// 客户端代码
public class Client {
    public static void main(String[] args) {
        ConcreteSubject subject = new ConcreteSubject();

        Observer obs1 = new ConcreteObserver("观察者1");
        Observer obs2 = new ConcreteObserver("观察者2");

        subject.registerObserver(obs1);
        subject.registerObserver(obs2);

        subject.setState("状态更新");
    }
}

// 组件接口
public interface Coffee {
    double cost();
    String description();
}

// 具体组件类
public class SimpleCoffee implements Coffee {
    public double cost() {
        return 5.0;
    }

    public String description() {
        return "简单咖啡";
    }
}

// 装饰者类
public abstract class CoffeeDecorator implements Coffee {
    protected Coffee decoratedCoffee;

    public CoffeeDecorator(Coffee coffee){
        this.decoratedCoffee = coffee;
    }

    public double cost(){
        return decoratedCoffee.cost();
    }

    public String description(){
        return decoratedCoffee.description();
    }
}

// 具体装饰者类
public class MilkDecorator extends CoffeeDecorator {
    public MilkDecorator(Coffee coffee){
        super(coffee);
    }

    public double cost(){
        return super.cost() + 1.5;
    }

    public String description(){
        return super.description() + ", 加牛奶";
    }
}

public class SugarDecorator extends CoffeeDecorator {
    public SugarDecorator(Coffee coffee){
        super(coffee);
    }

    public double cost(){
        return super.cost() + 0.5;
    }

    public String description(){
        return super.description() + ", 加糖";
    }
}

// 客户端代码
public class Client {
    public static void main(String[] args) {
        Coffee coffee = new SimpleCoffee();
        System.out.println(coffee.description() + " $" + coffee.cost());

        coffee = new MilkDecorator(coffee);
        System.out.println(coffee.description() + " $" + coffee.cost());

        coffee = new SugarDecorator(coffee);
        System.out.println(coffee.description() + " $" + coffee.cost());
    }
}

// 策略接口
public interface PaymentStrategy {
    void pay(int amount);
}

// 具体策略类 - 支付宝
public class AlipayStrategy implements PaymentStrategy {
    public void pay(int amount) {
        System.out.println("使用支付宝支付: $" + amount);
    }
}

// 具体策略类 - 微信支付
public class WeChatPayStrategy implements PaymentStrategy {
    public void pay(int amount) {
        System.out.println("使用微信支付: $" + amount);
    }
}

// 上下文类
public class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    public void setPaymentStrategy(PaymentStrategy strategy){
        this.paymentStrategy = strategy;
    }

    public void checkout(int amount){
        paymentStrategy.pay(amount);
    }
}

// 客户端代码
public class Client {
    public static void main(String[] args) {
        ShoppingCart cart = new ShoppingCart();

        // 使用支付宝支付
        cart.setPaymentStrategy(new AlipayStrategy());
        cart.checkout(100);

        // 使用微信支付
        cart.setPaymentStrategy(new WeChatPayStrategy());
        cart.checkout(200);
    }
}