class Invoice{     public void PRintInvoice(){         System.out.println("this is content");     } }  class Decorator extends Invoice{     protected Invoice ticket;     public Decorator (Invoice t){         ticket=t;     }     public void printInvoice(){         if(ticket!=null){             ticket.printInvoice();         }     } }  class HeaderDecorator extends Decorator{              public HeaderDecorator(Invoice t){         suPEr(t);     }     public void printInvoice(){         System.out.println("this is the header");         super.printInvoice();     }      }  class FooterDecorator extends Decorator{          public FooterDecorator(Invoice t){         super(t);     }     public void printInvoice(){                  super.printInvoice();         System.out.println("this is the footer");     }      }    public class zhuangshi {          public static void main(String[] args){         Invoice t =new Invoice();         Invoice ticket;         ticket=new FooterDecorator(new HeaderDecorator(t));         ticket.printInvoice();         System.out.println("=====================");         ticket=new FooterDecorator(new HeaderDecorator(new Decorator(null)));         ticket.printInvoice();     }  } 

结果