运用三种设计模式的程序

以下是一个运用了三种设计模式的程序示例：

// 工厂模式
public interface Animal {
    void makeSound();
}

public class Dog implements Animal {
    @Override
    public void makeSound() {
        System.out.println("汪汪汪！");
    }
}

public class Cat implements Animal {
    @Override
    public void makeSound() {
        System.out.println("喵喵喵！");
    }
}

public class AnimalFactory {
    public Animal createAnimal(String type) {
        if (type.equalsIgnoreCase("dog")) {
            return new Dog();
        } else if (type.equalsIgnoreCase("cat")) {
            return new Cat();
        } else {
            return null;
        }
    }
}

// 装饰器模式
public interface Component {
    void operation();
}

public class ConcreteComponent implements Component {
    @Override
    public void operation() {
        System.out.println("执行具体组件的操作");
    }
}

public class Decorator implements Component {
    private Component component;

    public Decorator(Component component) {
        this.component = component;
    }

    @Override
    public void operation() {
        component.operation();
    }
}

public class ConcreteDecorator extends Decorator {
    public ConcreteDecorator(Component component) {
        super(component);
    }

    @Override
    public void operation() {
        super.operation();
        addedBehavior();
    }

    private void addedBehavior() {
        System.out.println("为具体组件增加额外的操作");
    }
}

// 观察者模式
public interface Observer {
    void update(String message);
}

public interface Subject {
    void attach(Observer observer);
    void detach(Observer observer);
    void notify(String message);
}

public class ConcreteSubject implements Subject {
    private List<Observer> observers = new ArrayList<>();

    @Override
    public void attach(Observer observer) {
        observers.add(observer);
    }

    @Override
    public void detach(Observer observer) {
        observers.remove(observer);
    }

    @Override
    public void notify(String message) {
        for (Observer observer : observers) {
            observer.update(message);
        }
    }
}

public class ConcreteObserver implements Observer {
    private String name;

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

    @Override
    public void update(String message) {
        System.out.println(name + "收到消息：" + message);
    }
}

public class ObserverTest {
    public static void main(String[] args) {
        ConcreteSubject subject = new ConcreteSubject();
        Observer observer1 = new ConcreteObserver("张三");
        Observer observer2 = new ConcreteObserver("李四");
        subject.attach(observer1);
        subject.attach(observer2);
        subject.notify("新商品已上架");
    }
}

这个程序示例中，工厂模式用于创建不同种类的动物对象，装饰器模式用于给已有的组件增加额外的操作，观察者模式用于实现对象之间的消息通知。