How ro display array from constructor, using display method?

Question

I tried to use display method to print String array created in contstructor, but it doesn't work....

class Person{
    public String givenName;
    public String lastName;
    public int age;
    public String gender;
    public String citizenship;
    public String[] students;

    public Person(){
        String[] students = {"Jae Choi", "Su Wu", "Harry Potter"};
        this.students = students; 
        this.givenName = "Jae";
        this.lastName = "Choi";
        this.age = 27;
        this.gender = "male";
        this.citizenship = "Korea";
    }
}


class Student extends Person{
    public double[] marks;
    public String [] diciplines;
    public int i;

    public Student(){
        double marks[] = {95.0, 50.5, 75.0};
        String diciplines[] = {"Supplementary class", "Consultation", "Extra homework"};
            for (i = 0; i < marks.length; i++){
                this.marks = marks;
                System.out.println("Student name: " + this.students[i] + " / Mark: " + this.marks[i]);
            }
    }

public void displayDisciplines(){
        String diciplines[] = {"Supplementary class", "Consultation", "Extra homework"};
        for (int i = 0; i < diciplines.length; i++){
            System.out.println("Student name: " + this.students[i] + " / Dicipline: " + this.diciplines[i]);
        }
    }

public class Main{
    public static void main(String[] args) {
        Student student = new Student();
        student.averageMarks();
        student.displayDisciplines();
}

Exception in thread "main" java.lang.NullPointerException at Student.displayDisciplines(Student.java:41) at Main.main(Main.java:5)

this is the result.

Possible duplicate of [What is a NullPointerException, and how do I fix it?](https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — S.L. Barth is on codidact.com, Apr 02 '19 at 08:58
Sorry I haven't learned the super yet. Could let me know where I should put the super? — Jaehyuck Choi, Apr 02 '19 at 09:30
Into `Student`'s constructor. So, before `double marks[] = {95.0, 50.5, 75.0};`, add: `super();`. This will call the parent(Person)'s constructor and populate the local properties. More about it here: https://stackoverflow.com/questions/3767365/super-in-java . though it should be implicit. — briosheje, Apr 02 '19 at 09:32

Ashish Mahajan · Answer 1 · 2019-04-02T09:34:59.963

2

just edit your discipline line it will create new String Array which is not assigning class discipline field and you are printing class discipline field that's why this problem is coming.

so change

 String diciplines[] = {"Supplementary class", "Consultation", "Extra homework"};

to

 this.diciplines= new String[]{"Supplementary class", "Consultation", "Extra homework"};

it will run.

System.out.println("Student name: " + this.students[i] + " / Mark: " + this.marks[i]);

this.student is not valid at this position you have to student field to use that.

edited Apr 02 '19 at 09:34

answered Apr 02 '19 at 09:07

Ashish Mahajan

108
5

Sorry I missed it when I wrote this question. Actually, parents class has students. – Jaehyuck Choi Apr 02 '19 at 09:18
can you please update your question with parent class..it will be helpful – Ashish Mahajan Apr 02 '19 at 09:19
1

please remove line student.averageMarks(); there is no such a function like averageMarks(). – Ashish Mahajan Apr 02 '19 at 09:37

score 0 · Answer 2 · answered Apr 02 '19 at 09:05

0

It is not clear what this.students is in line. There is no such private member.

System.out.println("Student name: " + this.students[i] 
  + " / Dicipline: " + this.diciplines[i]);

answered Apr 02 '19 at 09:05

Anand Nath

31
4

Sorry I missed it when I wrote this question. Actually, parents class has students. – Jaehyuck Choi Apr 02 '19 at 09:18

How ro display array from constructor, using display method?

2 Answers2