观察者模式（Observer Pattern）

public abstract class Subject {
protected ArrayList observers<Observer> = new ArrayList();

public void attach(Observer observer) {
        observers.add(observer);    
}
public void detach(Observer observer) {
        observers.remove(observer);    
}
public abstract void notify( );
}

public class ConcreteSubject extends Subject {
    //实现通知方法
    public void notify() {
        for(Object obs:observers) {
            ((Observer)obs).update();
        }
    }	
}

public interface Observer {
    public void update();
}

public class ConcreteObserver implements Observer {

    public void update() {
        ……
    }
}

public interface Stocks {

    ArrayList<Observer> OBSERVERS = new ArrayList<>();
    void attach(Observer observer);
    void detach(Observer observer);
    void notifyInvestor();

}

public class Stock implements Stocks{

    private String stockName;
    private int price;

    public Stock(String stockName, int price) {
        this.stockName = stockName;
        this.price = price;
    }

    public Stock() {

    }

    public String getStockName() {
        return stockName;
    }

    public void setStockName(String stockName) {
        this.stockName = stockName;
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        this.price = price;
    }

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

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

    @Override
    public void notifyInvestor() {
        if(price >= 25*1.05 || price <= 25*0.95){
            System.out.println("通知：股票价格变动幅度超过5%！");
            for (Observer observer: OBSERVERS) {
                observer.upDate();
            }
        } else {
            System.out.println("股票价格变化幅度没有超过5%！！");
        }
    }
}

public interface Observer {

    void upDate();

}

public class ConcreteObserverA implements Observer {

    private String name;
    private Stock stock;

    public ConcreteObserverA(String name, Stock stock) {
        this.name = name;
        this.stock = stock;
    }

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

    @Override
    public void upDate() {
        System.out.println("通知"+name+":"+stock.getStockName()+"的股票价格变化幅度超过5%,股票价格为："+stock.getPrice());
    }
}

public class ConcreteObserverB implements Observer {

    private String name;

    public ConcreteObserverB(String name, Stock stock) {
        this.name = name;
        this.stock = stock;
    }

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

    private Stock stock;

    @Override
    public void upDate() {
        System.out.println("通知"+name+":"+stock.getStockName()+"的股票价格变化幅度超过5%,股票价格为："+stock.getPrice());
    }
}

public class ConcreteObserverC implements Observer {

    private String name;
    private Stock stock;

    public ConcreteObserverC(String name, Stock stock) {
        this.name = name;
        this.stock = stock;
    }

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

    @Override
    public void upDate() {
        System.out.println("通知"+name+":"+stock.getStockName()+"的股票价格变化幅度超过5%,股票价格为："+stock.getPrice());
    }
}

public class Test {

    public static void main(String[] args) {
        System.out.println("设计模式，2020006924，于鑫");
        Stock stock1 = new Stock("股票1",25);
        System.out.println(stock1.getStockName()+"的股价为："+stock1.getPrice());
        Observer obs1,obs2,obs3;
        obs1 = new ConcreteObserverA("smith",stock1);
        obs2 = new ConcreteObserverB("tom",stock1);
        obs3 = new ConcreteObserverC("李白",stock1);
        stock1.attach(obs1);
        stock1.attach(obs2);
        stock1.attach(obs3);
        stock1.setPrice(22);

        stock1.notifyInvestor();

    }

}