package chapter3;public class A {
public A() {
} public static void main(String argv[])
{ A a1 = new A();
B b1 = new B();
b1=(B)a1; }
}package chapter3;public class B extends A{
public B() {
}}
public A() {
} public static void main(String argv[])
{ A a1 = new A();
B b1 = new B();
b1=(B)a1; }
}package chapter3;public class B extends A{
public B() {
}}
A a1 = new B();
B b1 = new B();
b1=(B)a1;
子类的对象可当着父类的对象来使用,但反之则禁止。因为子类对父类加以扩充,因而包含有父类的成员。