java(第8章面向对象(多态性))new

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1、Java编程基础JavaJava编程基础编程基础——对象的多态杨巍yangwei@mail.neu.edu.cnJava编程基础多态性¢多态性¢方法的重载overload与覆写override¢对象的多态性¢具有继承关系的引用类型变量才能相互转换¢向上转型Upcasting¢子类对象Æ父类对象¢向下转型DownwardCast¢父类对象Æ子类对象2Employeeemployee=newManager();//legalemployee.department="Sales";//illegalpackagec

2、om.yangwei.up;classA{publicvoidfun1(){System.out.println("A-->fun1");}publicvoidfun2(){this.fun1();}}classBextendsA{publicvoidfun1(){System.out.println("B-->fun1");}publicvoidfun3(){System.out.println("B-->fun3");}}publicclassPolDemo1{publicstaticvoidmain(S

3、tring[]args){Bb=newB();Aa=b;a.fun1();}}packagecom.yangwei.up;classA{publicvoidfun1(){System.out.println("A-->fun1");}publicvoidfun2(){this.fun1();}}classBextendsA{publicvoidfun1(){System.out.println("B-->fun1");}publicvoidfun3(){System.out.println("B-->fun3

4、");}}publicclassPolDemo1{publicstaticvoidmain(String[]args){Bb=newB();Aa=b;a.fun3();}}packagecom.yangwei.up;classA{publicvoidfun1(){System.out.println("A-->fun1");}publicvoidfun2(){this.fun1();}}classBextendsA{publicvoidfun1(){System.out.println("B-->fun1")

5、;}publicvoidfun3(){System.out.println("B-->fun3");}}publicclassPolDemo1{publicstaticvoidmain(String[]args){Bb=newB();Aa=b;a.fun2();}}Java编程基础Upcasting上溯造型¢向上转型Upcasting¢子类对象Æ父类对象¢程序会自动完成¢向上转型损失了子类新扩展的属性和方法，仅剩下父类中声明过的属性和方法¢如果对象发生了向上转型关系后，所调用的方法一定是被子类覆盖过的方法7c

6、lassUpper{publicvoidf(){System.out.println("Upper.f()");}}classDownextendsUpper{publicvoidf(){System.out.println("Down.f()");}}classUpcastingtest{publicstaticvoidDoit(Upperu){u.f();}publicstaticvoidmain(String[]args){Downdown=newDown();Doit(down);}}classUpp

7、er{publicvoidf(){Java编程基础System.out.println("Upper.f()");}}classDownextendsUpper{publicvoidf(){System.out.println("Down.f()");}}classUpcastingtest{publicstaticvoidDoit(Downd){d.f();}publicstaticvoidmain(String[]args){Downdown=newDown();Doit(down);}9}classUp

8、per{publicvoidf(){System.out.println("Upper.f()");}Java编程基础}classDownextendsUpper{publicvoidf(){System.out.println("Down.f()");}}classDown1extendsUpper{publicvoidf(){System.out.println("Down1.f()");}}c