Is Java Collections add method immutable

Question

Sorry, I have corrected the logic a but , my ask was different . However it got clarified after referring to Java pass by reference, which create another reference variable while pushing it to stack or list or array.

public class Node {

    public int data;
    public Node left;
    public Node right;

}

public static void main(String[] args) {
        Node[] arr = new Node[10];
        Stack<Node> s = new Stack<Node>();
        List<Node> ls = new ArrayList<Node>();
        List<Integer> list = new ArrayList<Integer>();
        for(int i = 0; i <10; i++) {
            Node a = new Node();
            a.data = i;
            s.push(a); -> another reference to a is created and pushed
            ls.add(a); -> another reference to a is created and pushed
            arr[i] = a; -> -> another reference to a is created and pushed
            a = null; // referencing it to null, makes one of the references 
                      //to null
        }

        for(Node i : s) {
            System.out.println("result"+i.data); // prints 1-9
        }
        for(Node i : ls) {
            System.out.println("result"+i.data); // prints 1-9
        }
        System.out.println(arr.length); // prints 10
        for(Node i : arr) {
            System.out.println("result2"+i.data); // prints 1-9
        }

    }

What is the stack's push() or list's add() is making the object retain its value? even after I set its value to null. Is it something like a variable copy and Thanks.

The loop will raise an exception. You access to an out of bound index of the array : 10. — davidxxx, Feb 07 '18 at 21:14
Possible duplicate of [What causes a java.lang.ArrayIndexOutOfBoundsException and how do I prevent it?](https://stackoverflow.com/questions/5554734/what-causes-a-java-lang-arrayindexoutofboundsexception-and-how-do-i-prevent-it) — Savior, Feb 07 '18 at 21:15
@DAle no, the `for` loop should be a standard `for(int i = 0;i < 10; i++)`. — Kayaman, Feb 07 '18 at 21:19

davidxxx · Answer 1 · 2018-02-07T21:21:45.117

In the actual code, the loop will raise an exception as you access to an out of bound index of the array : 10.
About the printing issue in the second loop, you get a NPE because the first index of the array is not valued in your first loop : you start it to 1 but in the second loop you use a for each that iterates all indexes of the array : 0 included.
arr[0] being null, arr[0].data can only rise a NPE.

Morale : initialize your loop to 0 to fill an array :

for(int i = 0; i < 10; i++) {...}

It avoids this kind of error.

Is Java Collections add method immutable

1 Answers1