如何在Java中使用另一类的变量？

Question

I'm just working through a few things as practice for an exam I have coming up, but one thing I cannot get my head round, is using a variable that belongs to one class, in a different class.

I have a Course class and a Student class. Class course stores all the different courses and what I simply want to be able to do is use the name of the course, in class Student.

Here is my Course class:

public class Course extends Student
{
    // instance variables - replace the example below with your own
    private Award courseAward;
    private String courseCode;
    public String courseTitle;
    private String courseLeader;
    private int courseDuration;
    private boolean courseSandwich;

    /**
     * Constructor for objects of class Course
     */
    public Course(String code, String title, Award award, String leader, int duration, boolean sandwich)
    {
        courseCode = code;
        courseTitle = title;
        courseAward = award;
        courseLeader = leader;
        courseDuration = duration;
        courseSandwich = sandwich;

    }

}

And here is Student:

public class Student 
{
    // instance variables - replace the example below with your own
    private int studentNumber;
    private String studentName;
    private int studentPhone;
    private String studentCourse;

    /**
     * Constructor for objects of class Student
     */
    public Student(int number, String name, int phone)
    {
        studentNumber = number;
        studentName = name;
        studentPhone = phone;
        studentCourse = courseTitle;
    }

}

Am I correct in using ' extends ' within Course? Or is this unnecessary?

In my constructor for Student, I am trying to assign 'courseTitle' from class Course, to the variable 'studentCourse'. But I simply cannot figure how to do this!

Thank you in advance for your help, I look forward to hearing from you!

Thanks!

Answer 1

Am I correct in using 'extends' within Course? Or is this unnecessary?

Unfortunately not, if you want to know whether your inheritance is correct or not, replace extends with is-a . A course is a student? The answer is no. Which means your Course should not extend Student

A student can attend a Course , hence the Student class can have a member variable of type Course . You can define a list of courses if your model specifies that (a student can attend several courses).

Here is a sample code:

public class Student{
    //....
    private Course course;
    //...
    public void attendCourse(Course course){
       this.course = course;
    }
    public Course getCourse(){
       return course;
    }
}

Now, you can have the following:

Student bob = new Student(...);
Course course = new Course(...);
bob.attendCourse(course);

Answer 2

我认为课程不是学生，因此在这些课程之间进行继承可能不是一个好主意。

Answer 3

You have to declare them public.

A better way is the keep them private, and code a public getter for that variable. for example:

public Award getCourseAward(){
         return this.courseAward;
}

Answer 4

Course should not extend Student . If you want to access the courseTitle field of Course , you need to pass a reference to a Course object to the Student and then do course.CourseTitle.

Answer 5

You cannot access private attributes of a class from another, this is one of the main principles of OOP: encapsulation. You have to provide access method to those attribute, you want to publish outside the class. The common approach is setter/getters - getters only, if you want to have your class immutable. Look here: http://en.wikipedia.org/wiki/Mutator_method#Java_example

Answer 6

It does not make sense to arbitrarily extend classes. Student is not a Course or vice versa, so you cannot extend them like that.

What you need to do is:

create a Course first:

       Course aCourse = new Course(..);

create a Student:

       Student aStudent = new Student(..);

assign the Course to the Student:

       aStudent.setCourse(aCourse.title);

Answer 7

Extending Student with Couse because they are not of the same kind. Extending one class with another happens when specializing a more general (in a sense) one.
The solution would be to pass courseTitle as an argument of the Student constructor

Answer 8

There should be 3 separate objects here, a Course, a Student, and an Enrollment. An enrollment connects a Student to a Course, a Course has many Students, and a Student can enroll in many courses. None of them should extend each other.

Answer 9

Maybe you do not need to add the course name to student. What I would do is add Students to some datastructure in Course. This is cleaner and reduces the coupling between Course and Student. This would also allow you to have Students being in more than one course. For example:

public class Course extends Student{
    private Award courseAward;
    private String courseCode;
    public String courseTitle;
    private Student courseLeader;//change to a student Object
    private int courseDuration;
    private boolean courseSandwich;
    private Set<Student> students;//have course hold a collection of students

/**
 * Constructor for objects of class Course
 */
public Course(String code, String title, Award award, Student leader, int duration, boolean sandwich){
    courseCode = code;
    courseTitle = title;
    courseAward = award;
    courseLeader = leader;
    courseDuration = duration;
    courseSandwich = sandwich;
    this.students=new HashSet<Student>();
}

public boolean addStudent(Student student){
    return students.add(student);
}

public Set<Student> getStudents(){
    return students;
} 

}

Answer 10

First,

You are extending Student class in Course class, which means, student class gets all the coruse class properties. So, the student class does not have the courseTitle property.

Second, yes, it is unnesessary - you need to do the following:

public class Course
{
    private Award courseAward;
    private String courseCode;
    public String courseTitle;
    private String courseLeader;
    private int courseDuration;
    private boolean courseSandwich;

    public Course(String code, String title, Award award, String leader, int duration, boolean sandwich)
    {
        courseCode = code;
        courseTitle = title;
        courseAward = award;
        courseLeader = leader;
        courseDuration = duration;
        courseSandwich = sandwich;

    }

}

public class Student 
{
    private int studentNumber;
    private String studentName;
    private int studentPhone;

    // This is where you keep the course object associated to student
    public Course studentCourse;

    public Student(int number, String name, int phone, Course course)
    {
        studentNumber = number;
        studentName = name;
        studentPhone = phone;
        studentCourse = course;
    }  
}

Example usage would be something like this:

Course course = new Course("ASD", "TITLE", null, "ME", 50, true);   
Student student = new Student(1, "JOHN", "5551234", course);

And then, get the course information you need from student via, ie:

student.studentCourse.courseTitle;

Since now student.studentCourse will be a course object with all of its properties.

Cheers,

Answer 11

As mentioned, stay away from the "extends" for this. In general, you shouldn't use it unless the "is-a" relationship makes sense.

You should probably provide getters for the methods on the Course class:

public class Course {
   ...
   public String getTitle() {
       return title;
   }
}

And then if the Student class needs that, it would somehow get a hold of the course (which is up to you in your design), and call the getter:

public class Student {
   private Set<Course> courses = new HashSet<Course>();

   public void attendCourse(Course course) {
       courses.add(course);
   }

   public void printCourses(PrintStream stream) {
       for (Course course : courses) {
           stream.println(course.getTitle());
       }
   }
}

Answer 12

Here below find out the solution of your problem and if you want to check below code on your machine then create a file named Test.java and paste the below codes:

package com;

class Course
{
    private Award courseAward;
    private String courseCode;
    public String courseTitle;
    private String courseLeader;
    private int courseDuration;
    private boolean courseSandwich;


    public Course(String code, String title, Award award, String leader, int duration, boolean sandwich)
    {
        courseAward = award;
        courseCode = code;
        courseTitle = title;
        courseLeader = leader;
        courseDuration = duration;
        courseSandwich = sandwich;

    }

    public Award getCourseAward() {
        return courseAward;
    }

    public void setCourseAward(Award courseAward) {
        this.courseAward = courseAward;
    }

    public String getCourseCode() {
        return courseCode;
    }

    public void setCourseCode(String courseCode) {
        this.courseCode = courseCode;
    }

    public String getCourseTitle() {
        return courseTitle;
    }

    public void setCourseTitle(String courseTitle) {
        this.courseTitle = courseTitle;
    }

    public String getCourseLeader() {
        return courseLeader;
    }

    public void setCourseLeader(String courseLeader) {
        this.courseLeader = courseLeader;
    }

    public int getCourseDuration() {
        return courseDuration;
    }

    public void setCourseDuration(int courseDuration) {
        this.courseDuration = courseDuration;
    }

    public boolean isCourseSandwich() {
        return courseSandwich;
    }

    public void setCourseSandwich(boolean courseSandwich) {
        this.courseSandwich = courseSandwich;
    }
}

class Student 
{
    private int studentNumber;
    private String studentName;
    private int studentPhone;
    private Course studentCourse;
    /**
     * Constructor for objects of class Student
     */
    public Student(int number, String name, int phone, Course course)
    {
        studentNumber = number;
        studentName = name;
        studentPhone = phone;
        studentCourse = course;
    }

    public int getStudentNumber() {
        return studentNumber;
    }
    public void setStudentNumber(int studentNumber) {
        this.studentNumber = studentNumber;
    }
    public String getStudentName() {
        return studentName;
    }
    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }
    public int getStudentPhone() {
        return studentPhone;
    }
    public void setStudentPhone(int studentPhone) {
        this.studentPhone = studentPhone;
    }
    public Course getStudentCourse() {
        return studentCourse;
    }
    public void setStudentCourse(Course studentCourse) {
        this.studentCourse = studentCourse;
    }
}

class Award{
    private long awardId;
    private String awardName;

    Award(long awardId, String awardName){
        this.awardId = awardId;
        this.awardName = awardName;
    }

    public long getAwardId() {
        return awardId;
    }

    public void setAwardId(long awardId) {
        this.awardId = awardId;
    }

    public String getAwardName() {
        return awardName;
    }

    public void setAwardName(String awardName) {
        this.awardName = awardName;
    }
}

public class Test{
    public static void main(String ar[]){

        // use your all classes here


    }
}