Java - object reference replaaced in object arraylist before called on

Question

I am having a problem where when I'm debugging my code I notice that my object list array is replacing an object before a new object is added to the list array

Previous Research

I looked researched some similar situations this situation did not apply to my case. Why can't I add objects to my List<>?

// thought I would check an see if my list was not adding because of something I coded. However, this did not seem to be the situation

Java Object reference

This was a somewhat helpful post however my add student call already has the new keyword when asking for a new object.

Relevant Object list Code

 private MyStudent[] list;
    private int num = 0;
    private static final int GROW_BY = 2;
    public MyStudentList()
    {
        list = new MyStudent[GROW_BY];
        num = 0;

    }

    public boolean add(MyStudent inStudent)
        {
            int index = find(inStudent);
            if (index == -1)
            {
                list[num++] = inStudent;
                return true;
            }
            else
            return false;
        }   

private int find(MyStudent inStudent)
    {

        int index = -1;
        int test = 0;

        for (int i = 0;  i < list.length && index == -1; i++)
        {
            if(list[i] == null)
            {

                return index;
            }
            if (inStudent.getID().equals(list[i].getID()))
            {
                index = i;

            }
        }

        return index;
    }

Relevent object code

public MyStudent(String inID, String inLastName, String inFirstName,int inTotalCredits, double inTotalGradePoints)
    {
        ID = inID;
        firstName = inFirstName;
        lastName = inLastName;

        totalCredits = inTotalCredits;
        totalGradePoints = inTotalGradePoints;
    }

call from main

 MyStudent addStudent = new MyStudent("833006711", "James", "Butt", 106, 202);
        System.out.println(myList.add(addStudent)); // add the student
        myList.print();
        System.out.println(myList.add(addStudent));//student exists return false
        myList.print();
        addStudent = new MyStudent("261458460", "Josephine", "Darakjy", 37, 91.33); // here is where my code faults
        System.out.println(myList.add(addStudent));

When the new student replaces the old instance variable it replaces the reference in my MuStudentList. before i call the primative num to increment and add to objectlist What am i doing wrong?

in short i am trying to add a student(object) to my arraylist however, when i replace the referenced values with new values. it also replaces the values referenced in myStudentlist ( does this mean that i cannot dereference the object once it hits the list?)

The problem occurs when i call the constructor for the myStudent before i reach the find function. However since == shouldnt be used with strings i took the advice of fellow programmers and updated that as well.

Check the getId () comparison, these are strings and it is using == — KevinO
– KevinO, Commented May 6, 2017 at 17:04

Nishesh Pratap Singh · Accepted Answer · 2017-05-06 17:08:50Z

0

Try below changes in find function :

for (int i = 0;  i < list.length; i++)
{

    if (list[i]!=null && inStudent.getID().equals(list[i].getID()))
    {
        index = i;
        break;
    }
}

answered May 6, 2017 at 17:08

Nishesh Pratap Singh

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2 Comments

Nishesh Pratap Singh Over a year ago

When I changed the above logic, I was able to save two different MyStudent object in list. I was thinking that your issue is something like you are not able to save two different objects with same reference variable in the list.

PrincessLulu Over a year ago

yes that is the problem i am having. but it would seem like an extreme waste of memory if i couldnt do that. so it must be possible

PrincessLulu · Accepted Answer · 2017-05-06 19:05:38Z

0

Found the Answer on my own in the code where I set the parameters of the myStudent Object to static rather than just private.

Answer** Ensure that the private variables that mystudent object uses are not allowed to be changed at the class level.

private String lastName;
private String firstName;
private String ID;

Was

private static String lastName;
private static String firstName;
private static String ID;

answered May 6, 2017 at 19:05

PrincessLulu

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