设计模式之命令模式

命令模式将一个请求封装成一个对象，从而使您可以用不同的请求对客户进行参数化。命令模式的优点是降低了系统耦合度，新的命令很容易添加到系统中去；缺点是使用命令模式可能会导致某些系统有过多的具体命令类。

1. 创建一个命令接口；

public interface Order {
   void execute();
}
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2. 创建一个请求类；

class Stock {
    private String name = "ABC";
    private int quantity = 10;

    void buy(){
        System.out.println("Stock[Name:" + name + ",Quantity:" + quantity + "] bought.");
    }

    void sell(){
        System.out.println("Stock[Name:" + name + ",Quantity:" + quantity + "] sold.");
    }
}
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3. 创建命令接口的实现类；

/**
 * 买入股票指令
 */
public class BuyStock implements Order {
    private Stock stock;

    BuyStock(Stock stock){
        this.stock = stock;
    }

    @Override
    public void execute() {
        stock.buy();
    }
}

/**
 * 卖出股票指令
 */
public class SellStock implements Order {
    private Stock stock;

    SellStock(Stock stock){
        this.stock = stock;
    }

    @Override
    public void execute() {
        stock.sell();
    }
}
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4. 创建命令调用类；

public class Broker {

    private List<Order> orders = new ArrayList<>();

    void takeOrder(Order order){
        orders.add(order);
    }

    void placeOrders(){
        for(Order order:orders){
            order.execute();
        }

        orders.clear();
    }
}
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5. 使用命令调用类接收、执行命令。

public class CommandInstance {
    public static void main(String[] args) {
        Stock stock = new Stock();
        BuyStock buyStockOrder = new BuyStock(stock);
        SellStock sellStock = new SellStock(stock);
        Broker broker = new Broker();
        broker.takeOrder(buyStockOrder);
        broker.takeOrder(sellStock);
        broker.placeOrders();
    }
}
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