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 FootDecorator extends Decorator{ public FootDecorator（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 header 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!