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 modifier and path. The three categories do not try to put the same path, the self-design some of the challenges difficult. To strengthen their access type modifier and the relationship between the path of understanding.