PennPerson.class

public class Main {
    public static void main(String[] args) {
        PennPerson p = new Professor();
        p.sleep(6);
        Undergraduate u = new Undergraduate();
        // p.teach("CIS 120");
    }
}

public interface PennPerson {
    public void sleep(int hours);
}

public class Professor implements PennPerson {
    public void sleep(int hours) {
        System.out.println(Integer.toString(hours * 2) + "hours");
    }
    
    public void teach(String course) {
        System.out.println("teaching " + course);
    }
}

public abstract class Student implements PennPerson {
    private String[] coursesEnrolled;
    
    public Student() {
        coursesEnrolled = new String[5];
    }
    
    public void sleep(int hours) {
        System.out.println(Integer.toString(hours / 2) + "hours");
    }
    
    public void abstract enrolls(String course, int n);
}

public class Undergraduate extends Student {
    public Undergraduate() {
        super();
    }

    public void enrolls(String course, int n) {
        coursesEnrolled[n] = course;
    }
}

public class Graduate extends Student {
    
}