利用继承关系，设计名为Person、小学生PupilStudent、研究生GraduateStudent三个类，其中每个类都有toString方法返回各自的类型信息。PupilStudent有上学方法GoToSchool，GraduateStudent有从事项目方法DoProject；根据多态性，使用ArrayList链表将各个对象放入其中。利用循环取出链表中的元素，根据类型调用相应的方法。最后清空链表中的元素

代码如下（Java语言）：

Person类：

public class Person {
    @Override
    public String toString() {
        return "Person";
    }
}

PupilStudent类：

public class PupilStudent extends Person {
    public void goToSchool() {
        System.out.println("PupilStudent goes to school.");
    }
    
    @Override
    public String toString() {
        return "PupilStudent";
    }
}

GraduateStudent类：

public class GraduateStudent extends Person {
    public void doProject() {
        System.out.println("GraduateStudent does project.");
    }
    
    @Override
    public String toString() {
        return "GraduateStudent";
    }
}

测试代码：

import java.util.ArrayList;

public class Test {
    public static void main(String[] args) {
        ArrayList<Person> list = new ArrayList<>();
        list.add(new PupilStudent());
        list.add(new GraduateStudent());
        list.add(new PupilStudent());
        
        for (Person p : list) {
            System.out.println(p.toString());
            if (p instanceof PupilStudent) {
                ((PupilStudent)p).goToSchool();
            } else if (p instanceof GraduateStudent) {
                ((GraduateStudent)p).doProject();
            }
        }
        
        list.clear();
    }
}

输出结果：

PupilStudent
PupilStudent goes to school.
GraduateStudent
GraduateStudent does project.
PupilStudent
PupilStudent goes to school.