装饰器模式 Decorator

package basic;
class  Invoice{
    public  void  printInvoice(){
        System.out.println("this is the content of the invoice");
    }
}
class Deco extends Invoice{
    protected  Invoice ticket;
    public  Deco(Invoice t){
        ticket=t;
    }
    public  void printInvoice(){
        if(ticket!=null){
            ticket.printInvoice();;
        }
    }
}
class  HeadDeco extends Deco{
    public HeadDeco(Invoice t){
        super(t);
    }
    public  void printInvoice(){
        System.out.println("this is the head of the invoice");
        super.printInvoice();
    }
}
class  FootDeco extends Deco{
    public FootDeco(Invoice t){
        super(t);
    }
    public  void printInvoice(){
        super.printInvoice();
        System.out.println("this is the foot of the invoice");
    }
}
public class InvoiceDemo {
    public static void main(String[] args) {
        Invoice t=new Invoice();
        Invoice ticket;
        ticket=new HeadDeco(new FootDeco(t)) ;
        ticket.printInvoice();
        System.out.println("----------------");
        ticket=new FootDeco(new HeadDeco(new Deco(null)));
        ticket.printInvoice();
    }
}        

this is the head of the invoice
this is the content of the invoice
this is the foot of the invoice
----------------
this is the head of the invoice
this is the foot of the invoice