试题内容

阅读下列说明和java代码，将应填入  （n）  处的字句写在答题纸的对应栏内。
【说明】
某发票（lnvoice）由抬头（Head）部分、正文部分和脚注（Foot）部分构成。现采用装饰（Decorator）模式实现打印发票的功能，得到如图6-1所示的类图。

【java代码】 class invoice{
public void printInvoice(){
System.out.println ( "This is the content of the invoice!");
}
}
class Decorator extends Invoice {
protected Invoice ticket;
public Decorator(lnvoice t){
ticket = t; }
public void printInvoice(){
if(ticket != null)
（1） ;
}
}
class HeadDecorator extends Decorator{
public HeadDecorator(lnvoice t){
super(t);
}
public void printInvoice (){
Systent.out.println( "This is the header of the invoice! ");
（2） ;
}
}
class FootDecorator extends Decorator {
public FootDecorator(Invoice t){
super(t);
}
public void printlnvoice(){
（ 3）        ;
Systent.out.println( "This is the footnote of the invoice! ");
}
}
Class test {
public static void main(String[] args){
Invoice t =new Invioce();
Invoice ticket;
ticket=     （4）       ;
ticket.printInvoice();
Systent.out.println(“------------------“);
ticket=       （5）       ;
ticket.printInvoice();    } }
程序的输出结果为：
This is the header of the invoice!
This is the content of the invoice!
This is the footnote of the invoice!
----------------------------
This is the header of the invoice!
This is the footnote of the invoice!
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