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!