阅读以下说明和Java代码，填补代码中的空缺，将解答填入答题纸的对应栏内。
[说明]
在股票交易中，股票代理根据客户发出的股票操作指示进行股票的买卖操作。其类图如下图所示。相应的Java代码附后。

类图

[Java代码] importJava.util.ArrayList; importjava.util.List; ClaSS Stock{ private Stringname; private intquantity; publicStock(String name,int quantity){ thiS.name=name;this.quantity=quantity; } public void buy(){ System.out.println("[买进]:"+name+",数量:" +quantity);} public void sell(){System.out.println("[卖出]:"+name+",数量:" +quantity);} } interface Order { VOid execute(); } class BuyStock______ Order { private StockStock; publicBuyStock(Stock stock){______=stock; } public voidexecute(){ stock.buy(); } } //类SellStock实现和BuyStock类似,略 clasS Broker{ private List＜Order＞orderList=new ArrayList＜Order＞(); Dublic voidtakeOrder(______ Order){ orderList.add(order); } public voidplaceorders(){ for {______order:orderList) {order.execute(); } orderLiSt.clear(); } } public classStockCommand { public static voidmain(String[]args){ Stock aStock:newStock("股票A",10); Stock bStock=newStock("股票B",20); OrderbuyStockorder=new BuyStock(aStock); OrdersellStockOrder=new SellSt0Ck(bStoCk); Broker broker=newBroker(); broker.takeOrder(buyStockorder); broker.takeOrder(sellStockOrder); broker.______; } }