Java多态性——对象的多态性（向上、向下转型）

package test;class A{public void tell1(){System.out.println("A---tell1");}public void tell2(){System.out.println("A---tell2");}}class B extends A{public void tell1(){System.out.println("B---tell1");}public void tell3(){System.out.println("B---tell3");}}public class test {public static void main(String[] args) {//向上转型////B b = new B();////A a = b;//A a = new B();//a.tell1();//B重写了tell1方法//a.tell2();//向下转型A a = new B();B b = (B)a;b.tell1();b.tell2();b.tell3();}}