array of object -java-

Question

i have exercise that i Write a class CourseManager5 that stores a list of students’ objects in a given class. Each student has an id, name, score. The class allows the user to add a student, and display students’ data .. Here is the UML diagram: enter image description here The methods are:

• CourseManager5: a constructor that initializes the attributes and creates an array of students of size 100.
• getNStudents : returns the current number of students.
• addStudent: adds the student with the given object to the list. If course is full, it prints the error message: “ERROR: COURSE IS FULL” .
• displayStudent: displays all data of the student at index i. Write a main class called TestCourseManager5 with a main method that will do the following:
  • It creates a CourseManager5 object.
  • Then, it adds 3 students by reading their IDs, names, and scores from the user.
  • Then, it displays all students in class.

I write the program .. but i got problem with method displayStudent .. I don't why!!

One more thing: is method addStudent correct??

Here's my program:

import java.util.Scanner;
public class TestCourseManager5 {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        courseManager5 cm = new courseManager5();
        int num =0;
        for(int i =0; i < 3;i++){
            System.out.println("Please enter the ID, name, and score of student "+num+":");
            int id = s.nextInt();
            String name = s.next();
            double score = s.nextDouble();
            student su = new student(id, name, score);
            cm.addStudent(su);
            num++;}

        System.out.println("Student are: ");
        for(int i=0;i<cm.getNStudent();i++){
        cm.displayStudent(i);
        }}}

public class student {
    private int id;
    private String name;
    private double score;                   
    public student(int id, String name, double score){
        this.id = id;
        this.name = name;
        this.score = score;}

    public int getId(){
        return id;}

    public String getName(){
        return name;}

    public double getScore(){
        return score;}}

public class courseManager5 {
private student []students;
private int nStudent;
public static int MAX_SIZE = 100;

public courseManager5(){
    students = new student[MAX_SIZE];
    nStudent = 0;}

/addStudent adds the student with thegiven object to the list. If course is full, it prints the error message: “ERROR: COURSE IS FULL” is my method correct?/

public void addStudent (student newStudent){ 
    if(nStudent < MAX_SIZE)
        nStudent++;
    else
        System.out.println("ERROR: COURSE IS FULL");}

public int getNStudent(){
    return nStudent;}

public void displayStudent(int i){// my problem here !
        System.out.println(students[i].getId()+", "+students[i].getName()+", "+students[i].getScore());
    }
}

Answer 1

Your problem is in addStudent(). You never added the students to the array. So, that's resulting to null in displayStudent(). Try this:

public void addStudent (student newStudent){ 
    if(nStudent < MAX_SIZE)
        students[nStudent++] = newStudent;
    else
        System.out.println("ERROR: COURSE IS FULL");}

Answer 2

In your addStudent method your not adding the student passed in to the array. Try this:

students[nStudent] = newStudent;