设计3个类，分别是学生类Student，本科生类Undergraduate，研究生类Postgraduate，其中Student类是一个抽象类，它包含一个学生的基本信息如姓名、平均成绩等，及一个计算课程成绩等级的抽象方法。类Undergraduate和Postgraduate是其子类，它们计算课程成绩等级的方法有所不同（自己设计）。设计一个学生数组，既能放本科生对象又能放研究生对象。编写测试类测试以上功能。-Design of 3 categories, respectively, is a student of class Student Undergraduate, undergraduate, graduate class of Postgraduate, where Student class is an abstract class, it contains a student s basic information such as name, grade point average, and a computing course grade Abstract methods. Undergraduate and Postgraduate are its subclasses, which calculates the grade of different ways ( design ). Design of an array of students, not only can hold undergraduate and postgraduate object placed objects. Write the test class test above function.