java多态

Posted 2021-05-12 千里之外kb

1，父类接收子类对象不需要强转（自动转化）。子类接收父类对象需要强转

2，当子类执行super掉用父类的构造函数时，先执行完子类的方法（流程），在去调用父类的方法

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） ticket.printInvoice()；

} 
}

class HeadDecorator extends Decorator{
public HeadDecorator(lnvoice t){
super(t);
}

public void printInvoice (){
Systent.out.println( "This is the header of the invoice! ");
（2）ticket.printInvoice() ;
}
}

class FootDecorator extends Decorator {
public FootDecorator(Invoice t){
super(t);
}

public void printlnvoice(){
（ 3）ticket.printInvoice() ;
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）new FootDecorator(new HeadDecorator(t));  //new  HeadDecorator的时候就已经调用父类的super函数了，
ticket.printInvoice();　　　　　　　　　　　　　　　　　　　//再执行printInvoce函数时先由内之外执行，即先Decorator，HeadDecorator，FootDecorator
Systent.out.println(“------------------“);
ticket= new FootDecorator(new HeadDecorator(new Decorator(null)));//new Decorator的时候传null值进去，再到new HeadDecorator的时候调用父类
ticket.printInvoice();　　　　　　　　　　　　　　　　　　　　　　　　　　　//同上
}
}
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!