建议：此程序最好放到计算机上调试，不要仅凭感觉想，并注意访问权限修饰符与路径的关系。尝试将三个类不放在相同的路径下，自我设计一些挑战难题。强化自己对类的访问权限修饰符与路径的关系的理解。-Recommendations: the best place this program on your computer debugging, do not want to feel alone, and attention to access to the relationship between modi