补全以下代码private String cid;// Course id, e.g., CS110. private String name;// Course name, e.g., Introduce to Java Programming. private Integer credit;// Credit of this course private GradingSchema gradingSchema; //Grading schema of this course // enum GradingSchema{FIVE_LEVEL, PASS_FAIL} private Integer capacity;// Course capacity. private Integer leftCapacity;// Course capacity left. You should update the left capacity when enrolling students. private Set<Timeslot> timeslots;// One course may have one or more timeslots. e.g., a lecture in Monday's 10:20-12:10, and a lab in Tuesday's 14:00-15:50. public Course(String cid, String name, Integer credit, GradingSchema gradingSchema, Integer capacity) // constructor public void addTimeslot(Timeslot timeslot) //Record a timeslot for this course private Integer id;// A unique student id, should be an 8-digit integer: Undergraduates' ids should start with 1; Postgraduates' ids should start with 3. e.g., 12213199. private String name;// Student’s name private Map<Course, Grade> courses;// Enrolled courses, using Map structure to store course and its grade as a pair. Grade is an enum type enum Grade{PASS,FAIL,A,B,C,D,F}with an attribute: Double gradePoint protected Student(Integer id, String name) // constructor public abstract boolean canGraduate() // Checks if this student satisfies all the graduating conditions. Hint: you are allowed to change this abstract method into non-abstract to check if the student satisfies the common graduation conditions. public void enroll(Course course) // Tries to enroll the course, do some checks before enrolling. public void recordGrade(Course course, Grade grade)// Records the grade of a course that is current learning. public double getGpa() // Calculates the GPA for this student. public UndergraduateStudent(Integer id, String name)// constructor public boolean canGraduate() //Additional graduating conditions for undergraduate students public PostgraduateStudent(Integer id, String name)// constructor public boolean canGraduate() //Additional graduating conditions for postgraduate students

完整代码如下：

import java.util.*;

public class Course {
    private String cid;// Course id, e.g., CS110.
    private String name;// Course name, e.g., Introduce to Java Programming.
    private Integer credit;// Credit of this course
    private GradingSchema gradingSchema; //Grading schema of this course // enum GradingSchema{FIVE_LEVEL, PASS_FAIL}
    private Integer capacity;// Course capacity.
    private Integer leftCapacity;// Course capacity left. You should update the left capacity when enrolling students.
    private Set<Timeslot> timeslots;// One course may have one or more timeslots. e.g., a lecture in Monday's 10:20-12:10, and a lab in Tuesday's 14:00-15:50.

    public Course(String cid, String name, Integer credit, GradingSchema gradingSchema, Integer capacity) {
        this.cid = cid;
        this.name = name;
        this.credit = credit;
        this.gradingSchema = gradingSchema;
        this.capacity = capacity;
        this.leftCapacity = capacity;
        this.timeslots = new HashSet<>();
    }

    public void addTimeslot(Timeslot timeslot) {
        this.timeslots.add(timeslot);
    }
}

class Student {
    private Integer id;// A unique student id, should be an 8-digit integer: Undergraduates' ids should start with 1; Postgraduates' ids should start with 3.   e.g., 12213199.
    private String name;// Student’s name
    private Map<Course, Grade> courses;// Enrolled courses, using Map structure to store course and its grade as a pair. Grade is an enum type enum Grade{PASS,FAIL,A,B,C,D,F}with an attribute: Double gradePoint

    public Student(Integer id, String name) {
        this.id = id;
        this.name = name;
        this.courses = new HashMap<>();
    }

    public abstract boolean canGraduate(); // Checks if this student satisfies all the graduating conditions. Hint: you are allowed to change this abstract method into non-abstract to check if the student satisfies the common graduation conditions.

    public void enroll(Course course) {
        if (course.leftCapacity > 0) {
            this.courses.put(course, null);
            course.leftCapacity--;
        }
    }

    public void recordGrade(Course course, Grade grade) {
        if (this.courses.containsKey(course)) {
            this.courses.put(course, grade);
        }
    }

    public double getGpa() {
        double points = 0.0;
        int credits = 0;
        for (Map.Entry<Course, Grade> entry : this.courses.entrySet()) {
            Course course = entry.getKey();
            Grade grade = entry.getValue();
            if (grade != null) {
                double gradePoint = grade.gradePoint;
                int credit = course.credit;
                points += gradePoint * credit;
                credits += credit;
            }
        }
        return credits == 0 ? 0 : points / credits;
    }
}

class UndergraduateStudent extends Student {
    public UndergraduateStudent(Integer id, String name) {
        super(id, name);
    }

    public boolean canGraduate() {
        double gpa = this.getGpa();
        int credits = 0;
        for (Map.Entry<Course, Grade> entry : this.courses.entrySet()) {
            Course course = entry.getKey();
            int credit = course.credit;
            credits += credit;
        }
        return gpa >= 2.0 && credits >= 120;
    }
}

class PostgraduateStudent extends Student {
    public PostgraduateStudent(Integer id, String name) {
        super(id, name);
    }

    public boolean canGraduate() {
        double gpa = this.getGpa();
        int credits = 0;
        for (Map.Entry<Course, Grade> entry : this.courses.entrySet()) {
            Course course = entry.getKey();
            int credit = course.credit;
            credits += credit;
        }
        return gpa >= 3.0 && credits >= 30;
    }
}

class Timeslot {
    private String day;// Day of the week, e.g., Monday, Tuesday, etc.
    private String startTime;// Start time of the timeslot, e.g., 10:20.
    private String endTime;// End time of the timeslot, e.g., 12:10.

    public Timeslot(String day, String startTime, String endTime) {
        this.day = day;
        this.startTime = startTime;
        this.endTime = endTime;
    }
}

enum Grade {
    PASS("Pass", 4.0),
    FAIL("Fail", 0.0),
    A("A", 4.0),
    B("B", 3.0),
    C("C", 2.0),
    D("D", 1.0),
    F("F", 0.0);

    public String name;
    public double gradePoint;

    Grade(String name, double gradePoint) {
        this.name = name;
        this.gradePoint = gradePoint;
    }
}

enum GradingSchema {
    FIVE_LEVEL,
    PASS_FAIL
}