阅读下列说明和java代码，将应填入 （n） 处字句写在答题纸对应栏内。
【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）模式实现打印发票功能，得到如图6-1所示类图。
【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!
（1） ticket.printInvoice()
（2） ticket.printInvoice()
（3） ticket.printInvoice()
（4） new FootDecorator(new
