Question about Objects inside and Object (Visualization request)

Question

class Eco {
    Eco e;

    public static void main(String[] args){
        Eco e1 = new Eco();
        Eco e2 = new Eco();
        Eco e3 = new Eco();

        e3.e = e2;
        e1.e = e3;
        e2 = null;
        e3 = null;
        e2.e = e1;
        e1 = null;
    }

}

Could someone visualize how the objects interact? This is a mocking question for my upcoming OCA exam and even if I do understand this topic, I still have some trouble actually visualizing what is actually happening. I did find the Error that happens if we reach e2.e = e1; since e2 was already null. I hope I made myself clear enough to understand what my question is :). Is it more like this:

[e1] --------> [e1 Object] ---------> [e Object]

[e2] --------> [e2 Object] ---------> [e Object]

[e3] --------> [e3 Object] ---------> [e Object]

or is it more like:

[e1] --------> [e1 Object|e Object]

[e2] --------> [e2 Object|e Object]

[e3] --------> [e3 Object|e Object]

I really hope I made myself clear here :D

let me know if I could be more specific.

Please strive to post well-formatted and easy-to-read code. No sense in making code harder to understand than it already is. — Hovercraft Full Of Eels, Feb 27 '21 at 13:01
I've fixed your code's formatting for you this time, but in the future, again, please put in this slight but important effort yourself. — Hovercraft Full Of Eels, Feb 27 '21 at 13:03
1. thanks for formatting it! 2. I just copied it from my mocking question, but I should have taken the Eco e to the top since it doesn't make a difference. Sorry! — Loskyll, Feb 27 '21 at 13:04
What is `[e Object]`? Do you mean the field `e` of the `Eco` class? That is part of the `Eco` object and itself a reference to another `Eco` object. — Sotirios Delimanolis, Feb 27 '21 at 13:05
More important is the code indentation. Code without any indents is impossible to read — Hovercraft Full Of Eels, Feb 27 '21 at 13:05
I'm pretty new to this site and didn't know how to do it. I will look out for it the next time — Loskyll, Feb 27 '21 at 13:06
https://stackoverflow.com/questions/32010172/what-is-the-difference-between-a-variable-object-and-reference — Sotirios Delimanolis, Feb 27 '21 at 13:06
There really is little "interaction" here. Just an object that holds a reference to another object of the same type. I think that the question is trying to make sure that you understand the difference between a reference variable and a reference — Hovercraft Full Of Eels, Feb 27 '21 at 13:13
They are asking about 'Question no 8' in this article https://www.javamadesoeasy.com/2018/07/garbage-collections-scjp-ocjp.html — the Hutt, Feb 27 '21 at 13:19

score 0 · Answer 1 · answered Feb 27 '21 at 13:14

The code

e3.e = e2;
e1.e = e3;

gives you linking like this e1.e -> e3.e -> e2 Making e2 and e3 null doesn't make them eligible for garbage collection cos they are raceable through e1.
only making e1=null makes all of them eligible for garbage collection.
but the code throws NullpointerException at e2.e = e1, which is the right answer.

Question about Objects inside and Object (Visualization request)

1 Answers1