Putting a class inside another list in another class

Question

so basically i want to put the class "GradeBook", which contains 2 arraylists, inside another class "course". I create a setCourse method to link both class together but when I tried to put in the GradeBook inside a list "gradebooks" inside "course" class, there's a NullPointerException. Why can't I put this class i the list.

public class GradeBook {

 private List<Double> assignmentScores;
 private List<Double> quizScores;
 private Course course;

 public GradeBook() {
    assignmentScores = new ArrayList<>();
    quizScores = new ArrayList<>();
 }
 public void setCourse(Course course) {
    this.course = course;
 }

public class Course {

private List<GradeBook> gradeBooks;

public void addGradebook(GradeBook gradebook) {
    gradebook.setCourse(this);
    gradeBooks.add(gradebook);
}

because `gradeBooks` is `null`. That’s what a `NullPointerException` tells you. By looking at the stack trace, you see where it happened, which provides you a hint about what could have been `null`. — Holger, Feb 07 '17 at 16:29

secondbreakfast · Answer 1 · 2017-02-07T16:31:04.110

2

You need to initialize gradeBooks. You are getting the null pointer because gradeBooks is null when you try to add a grade book. As an example, you could initialize it in a constructor of the Course class.

public Course() {
    gradeBooks = new ArrayList<>();
}

public void addGradebook(GradeBook gradebook) {
    gradebook.setCourse(this);
    gradeBooks.add(gradebook);
}

edited Feb 07 '17 at 16:31

answered Feb 07 '17 at 16:29

secondbreakfast

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Putting a class inside another list in another class

1 Answers1