Passing ArrayList in a object

Question

Hello I've got a problem with this program, it should store student information into an object of type Student . Stored information: last name, grade and votes. Votes are stored in an ArrayList of Integer . Every time I create a new object of type Student and I add it to an ArrayList of type Student (it stores all the students of a school), it keeps adding the new votes that I enter to the previous Student object created that it is already stored in the ArrayList .

EXAMPLE: I add a Student to the students ArrayList , I give in input freddy, 5b and 123 then i check the students ArrayList and it contains the student that i have already added: freddy,5b and 123. I add another student I give in input josh, 4t and 1234 I check

Why does it modify the object that is already created and stored in the ArrayList ? How can i fix it?

Here's the code:

public class Student {
    private String lastname;
    private String grade; // example "4b" 
    private ArrayList<Integer> student_votes;  


    public Student(String lastname, String grade, ArrayList<Integer> student_votes) {
        this.lastname=lastname;
        this.grade=grade;
        this.student_votes=student_votes; 
    }

    public ArrayList getVotes() {
        return student_votes;
    }

    public String getLastname() {
        return lastname;
    }

    public String getGrade() {
        return grade;
    }

    public String toString() {
        return lastname+" "+grade+" "+getVotes();
    }

    public void print_student (Student student) {
        System.out.println(student);
    }

    public static void print_students(ArrayList<Student> students) {
        for(Student s : students) {
            System.out.print(s);
        }
        System.out.println("");
    }

    public static void menu() {
        System.out.println("\nPress 1 to add a student\nPress 2 to remove a student\nPress 3 to print the classroom\nPress 4 to exit");
    }

    public static void main(String[] args) {
        int choice, nv=0, i=0,average=0;
        Boolean exit=false;
        ArrayList<Student> students = new ArrayList<Student>();
        ArrayList<Integer> votes = new ArrayList<Integer>();
        String lastname = new String();
        String grade = new String();

        Scanner sc = new Scanner(System.in);
        Scanner st = new Scanner(System.in);
        do { 
            menu();
            choice=sc.nextInt();

            switch (choice) {
                case 1:  System.out.println("Enter your lastname:");
                         lastname=st.nextLine();   
                         System.out.println("Enter your grade:");
                         grade=st.nextLine();

                         System.out.println("Enter the amount of votes");
                         nv=sc.nextInt();



                         for(i=0;i<nv;i++) {
                             System.out.println("Enter vote n:"+(i+1));                 
                             votes.add(sc.nextInt());
                         }

                         students.add(new Student(lastname,grade,votes));
                         System.out.println("student added!");

                         break;

                case 2:  System.out.println("Enter student position: ");
                         nv = sc.nextInt();
                         students.remove(nv-1);
                         break;

                case 3:  print_students(students);
                         break;

                case 4: exit = true;

            }
        } while (exit==false);
    }
}

Answer 1

You are using the same ArrayList object for each Student . The Student does not contain a copy of the list, but rather a pointer to the same list. If you keep adding to the list through any one pointer, all such pointers will be affected.

You should create a new ArrayList for each student and pass it into the constructor.

Answer 2

The problem lies into your logic of inputting the votes, you are using one ArrayList which collects all votes which will be given to any student.

When you pass an object, you don't copy the object but you give the reference to the object, meaning that with your logic all students will have the same reference to the votes ArrayList in your main. This can be easily solved by creating a new ArrayList for each student:

System.out.println("Enter the amount of votes");
nv=sc.nextInt();
final ArrayList<Integer> votes = new ArrayList<>();
for(i=0;i<nv;i++)
{
    System.out.println("Enter vote n:"+(i+1));
    votes.add(sc.nextInt());
}

This will fix it for you, but there really is no point by using an ArrayList in my opinion, as you will know the amount votes. Then the code will be this, however Student needs to be modified to accept an array:

System.out.println("Enter the amount of votes");
nv=sc.nextInt();
final int[] votes = new int[nv];
for(i=0;i<nv;i++)
{
    System.out.println("Enter vote n:"+(i+1));
    votes[i] = sc.nextInt();
}

Answer 3

You are using same ArrayList for votes for every student, every student has the same list of votes with the same votes (when you add the list to student-1 with lets say 2 votes, and then you add the list to student-2 with 3 votes, then both students will have 5 votes now), change it like this:

votes = new ArrayList<Integer>();
for(i=0;i<nv;i++)
{
  System.out.println("Enter vote n:"+(i+1));
  votes.add(sc.nextInt());
}

Should be fine now. Also you are using 2 scanners which is not necessary, 1 is enough.