练习1：/* Write a class named Outer that contains an inner class named Innet.
* Add a method to Outer that returns an object of type Inner. In main(),
* create and initialize a reference to an Inner.
练习1:/* Create a class with a main(0 that throws an object of class Exception
* inside a try block. Give the constructor for Exception a String argument.
* Catch the exception inside a catch clause and...
练习1：// Use Holder3 with the typeinfo.pets library to show that a Holder3 that is
// specified to hold a base type can also hold a derived type.