public class GradeBookApp4 {
	public static void main(String args[]) {
		Student Jie = new Student ("Jie");
		Student Albert = new Student ("Albert");
		Student Alex = new Student ("Alex"); //++++++++++

		Teacher Nick = new Teacher ("Nick");

		Albert.setEmail("albert@gmail.com"); //++++++++++
		Jie.setEmail("jie@gmail.com");
		Nick.setEmail("nick@gmail.com");

		Member[] members = {Albert, Jie, Nick}; //++++++++++
		for (Member m: members)
			m.showInfo();

		Course Java = new Course ("Java", 3);
		Course Python = new Course ("Python", 3);

		Nick.offer(Java);
		Nick.offer(Python);

		Jie.takeCourse(Java);
		Albert.takeCourse(Java);
		Alex.takeCourse(Java); //++++++++++

		Nick.score(Java, Jie, 100);
		Nick.score(Java, Albert, 98);
		Nick.score(Java, Alex, 20); //++++++++++

		// Jie.showGrade();
		// Albert.showGrade();
		// Alex.showGrade(); //++++++++++
		Java.showCourseInfo();
	}
}


class Course {
	String cName;
	private int degree;
	Student[] students = new Student[10];
	int studentCount = 0;
	Teacher teacher = new Teacher("None");
	int [] grades = new int[10];
	int scoreCount = 0; //++++++++++
	double sum=0; //++++++++++
	double average=0; //++++++++++

	public Course (String name, int degree) {
		this.cName = name;
		this.degree = degree;
	}

	public void registeredBy(Student s) {
		if (studentCount <= 9)
			students[studentCount++] = s;
		else
			System.out.println("Students overflow in a class");
	}

	public void showCourseInfo() {
		System.out.println("Course: "+ cName);
		System.out.println("-- Teacher: " + teacher.name); //*****
		String s = "";
		String g = "";
		for (int i=0; i<studentCount ; i++) {
			s += students[i].name + ", "; //*****
			g += Integer.toString(grades[i]) + ", ";
		}
		System.out.println("-- Students: " + s);
		System.out.println("-- Grades: " + g);
		System.out.println("-- Average: "+ average); //++++++++++
	}

	public void setTeacher(Teacher t) {
		teacher = t;
	}

	public boolean takenBy(Student s) {
		for (int i=0; i<studentCount ; i++) {
			if (students[i] == s) {
				return true;
			}
		}
		return false;
	}

	public void score(Student s, int g) {
		int idx = getIndex(s);
		if (idx == -1)
			System.out.println(s.name + " is not in " + cName);
		else {
			grades[idx] = g;
			scoreCount++; 	//++++++++++
			sum += g;
			average = sum/scoreCount;
		}
	}

	private int getIndex(Student s) {
		for (int i=0; i<studentCount; i++) {
			if (students[i] == s) {
				return i;
			}
		}
		return -1;
	}

	public int getGrade(Student s) {
		int idx = getIndex(s);
		if (idx != -1) {
			return grades[idx];
		}
		return idx;
	}
}

class Teacher extends Member { //*****
	// String tName; //----------
	// private String email;
	Course[] courses = new Course[10];
	int courseCount = 0;

	public Teacher(String name) {
		super(name); //*****
	}
	// public void setEmail(String e) {
	// 	this.email = e;
	// }
	public void offer(Course c) {
		if (courseCount <= 9) {
			courses[courseCount++] = c;
			c.setTeacher(this);
		}
		else
			System.out.println("Offer too many courses");
	}

	public void showCourse() {
		for (Course c: courses) {
			if (c != null)
				System.out.println(c.cName);
			else
				break;
		}
	}

	public void score(Course c, Student s, int g) {
		if (offeredByMe(c) && c.takenBy(s)) {
			c.score(s, g);
		}
	}

	private boolean offeredByMe(Course c) {
		for (int i=0; i<courseCount; i++) {
			if (courses[i] == c) {
				return true;
			}
		}
		return false;
	}
}

class Student extends Member { //*****
	Course[] courses = new Course[10];
	// String sName; //----------
	// private String email;
	int courseCount = 0;
	public Student (String name){
		super(name); //*****
	}
	public void takeCourse(Course c) {
		if (courseCount <=9) {
			courses[courseCount++] = c;
			c.registeredBy(this);
		}
		else
			System.out.println("Hei, you take too many courses");
	}
	public void showGrade() {
		System.out.println("The grades of student " + name); //*****
		for (int i=0; i<courseCount; i++) {
			Course c = courses[i];
			String gString = "no grade";
			int g = c.getGrade(this);
			if (g != -1) gString = Integer.toString(g);
			System.out.println("-- " + c.cName + ": " + gString);
		}
	}
}

abstract class Member { //++++++++++
	String name;
	private String email;
	public Member(String name) {
		this.name = name;
	}
	public void setEmail(String e) {
		this.email = e;
	}
	public void showInfo() { //++++++++++
		System.out.println(name + ", email: " + email);
	}
}



//******************************************************************



/*
INTERFACE and its implementation
* an Instructor interface define the "teach" behavior
* Teacher or IndustryExpert may server as an instructor
* all instructors are required to set their Qualification

* OCP principle
* when we have different types of qualification, we use "extension", not
  modify the code


* add an interface Instructor, methods: setQualification(Qualification)
	and showQualification()
* add a new class IndustryExpert, to implement the Instructor
* modify Teacher to implement the Instructor
* add a Qualification subclass Certification, which requires the
	year getting the certification.
* add a static method to show all instructors
*/

public class GradeBookApp5 {
	public static void main(String args[]) {
		Student Jie = new Student ("Jie");
		Student Albert = new Student ("Albert");
		Student Alex = new Student ("Alex");

		Teacher Nick = new Teacher ("Nick");

		IndustryExpert Peter = new IndustryExpert("Peter"); //++++++++++
		Nick.setQualification(new Qualification("IECS Ph.D")); //++++++++++
		Peter.setQualification(new Certification(2000, "Cisco")); //++++++++++
		Instructor tutors[] = {(Instructor)Nick, (Instructor)Peter}; //++++++++++
		System.out.println("\n=== INSTRUCTORS QUALIFICATION ==="); //++++++++++

		for (Instructor i: tutors)
			i.showQualification();

		Albert.setEmail("albert@gmail.com");
		Jie.setEmail("jie@gmail.com");
		Nick.setEmail("nick@gmail.com");

		Member[] members = {Albert, Jie, Nick};
		System.out.println("\n=== MEMBERS ==="); //++++++++++
		for (Member m: members)
			m.showInfo();

		Course Java = new Course ("Java", 3);
		Course Python = new Course ("Python", 3);

		Nick.offer(Java);
		Nick.offer(Python);

		Jie.takeCourse(Java);
		Albert.takeCourse(Java);
		Alex.takeCourse(Java);

		Nick.score(Java, Jie, 100);
		Nick.score(Java, Albert, 98);
		Nick.score(Java, Alex, 20);

		Course[] courses = {Java, Python};
		showCourses(courses);
	}

	public static void showCourses(Course[] courses) { //++++++++++
		System.out.println("\n=== COURSES ===");
		for (Course c: courses)
			c.showCourseInfo();
	}

}

Code
class Course {
	String cName;
	private int degree;
	Student[] students = new Student[10];
	int studentCount = 0;
	Teacher teacher = new Teacher("None");
	int [] grades = new int[10];
	int scoreCount = 0;
	double sum=0;
	double average=0;

	public Course (String name, int degree) {
		this.cName = name;
		this.degree = degree;
	}

	public void registeredBy(Student s) {
		if (studentCount <= 9)
			students[studentCount++] = s;
		else
			System.out.println("Students overflow in a class");
	}

	public void showCourseInfo() {
		System.out.println("Course: "+ cName);
		System.out.println("-- Teacher: " + teacher.name);
		String s = "";
		String g = "";
		for (int i=0; i<studentCount ; i++) {
			s += students[i].name + ", ";
			g += Integer.toString(grades[i]) + ", ";
		}
		System.out.println("-- Students: " + s);
		System.out.println("-- Grades: " + g);
		System.out.println("-- Average: "+ average);
	}

	public void setTeacher(Teacher t) {
		teacher = t;
	}

	public boolean takenBy(Student s) {
		for (int i=0; i<studentCount ; i++) {
			if (students[i] == s) {
				return true;
			}
		}
		return false;
	}

	public void score(Student s, int g) {
		int idx = getIndex(s);
		if (idx == -1)
			System.out.println(s.name + " is not in " + cName);
		else {
			grades[idx] = g;
			scoreCount++;
			sum += g;
			average = sum/scoreCount;
		}
	}

	private int getIndex(Student s) {
		for (int i=0; i<studentCount; i++) {
			if (students[i] == s) {
				return i;
			}
		}
		return -1;
	}

	public int getGrade(Student s) {
		int idx = getIndex(s);
		if (idx != -1) {
			return grades[idx];
		}
		return idx;
	}
}

class Teacher extends Member implements Instructor {  //*****
	Course[] courses = new Course[10];
	int courseCount = 0;
	Qualification qualification; //++++++++++

	public Teacher(String name) {
		super(name);
	}

	public void offer(Course c) {
		if (courseCount <= 9) {
			courses[courseCount++] = c;
			c.setTeacher(this);
		}
		else
			System.out.println("Offer too many courses");
	}

	public void showCourse() {
		for (Course c: courses) {
			if (c != null)
				System.out.println(c.cName);
			else
				break;
		}
	}

	public void score(Course c, Student s, int g) {
		if (offeredByMe(c) && c.takenBy(s)) {
			c.score(s, g);
		}
	}

	private boolean offeredByMe(Course c) {
		for (int i=0; i<courseCount; i++) {
			if (courses[i] == c) {
				return true;
			}
		}
		return false;
	}

	public void setQualification(Qualification q) { //++++++++++
		this.qualification = q;
	}

	public void showQualification() { //++++++++++
		System.out.println(name + " is a teacher because " + qualification);
	}
}

class Student extends Member {
	Course[] courses = new Course[10];

	int courseCount = 0;
	public Student (String name){
		super(name); //*****
	}
	public void takeCourse(Course c) {
		if (courseCount <=9) {
			courses[courseCount++] = c;
			c.registeredBy(this);
		}
		else
			System.out.println("Hei, you take too many courses");
	}
	public void showGrade() {
		System.out.println("The grades of student " + name);
		for (int i=0; i<courseCount; i++) {
			Course c = courses[i];
			String gString = "no grade";
			int g = c.getGrade(this);
			if (g != -1) gString = Integer.toString(g);
			System.out.println("-- " + c.cName + ": " + gString);
		}
	}
}

abstract class Member {
	String name;
	private String email;
	public Member(String name) {
		this.name = name;
	}
	public void setEmail(String e) {
		this.email = e;
	}
	public void showInfo() {
		System.out.println(name + ", email: " + email);
	}
}

interface Instructor { //++++++++++
	public void setQualification(Qualification q);
	public void showQualification();
}

class Qualification { //++++++++++
	String description;
	public Qualification(String desc) {
		this.description = desc;
	}
	public String toString() {
		return description;
	}
}

class Certification extends Qualification { //++++++++++
	public Certification(int year, String desc) {
		super(desc + " " + Integer.toString(year));
	}
}

class IndustryExpert implements Instructor { //++++++++++
	Qualification q;
	String name;
	public IndustryExpert(String name) {
		this.name = name;
	}
	public void setQualification(Qualification q) {
		this.q = q;
	}
	public void showQualification() {
		System.out.println(name + " is qualified to be an instructor because " + q);
	}
}