看下面这个例子：
class SuperClass...{
void method()...{
System.out.println("method() of SuperClass!");
}
} //父类
class SubClass extends SuperClass...{ void method()...{ System.out.println("method() of SubClass"); }} //子类public class MainClass...{ public static void main(String[] args)...{ SuperClass a1=new SuperClass(); SubClass b1=new SubClass(); SuperClass a2=new SubClass(); //SubClass b2=new SuperClass(); //编译出错。 a1.method(); b1.method(); a2.method(); //b2.method(); }}程序输出：
method() of SuperClass!
method() of SubClass
method() of SubClass