Java ArrayList independent copy

Question

I used the method below to make a copy of a list, as you can see the output, they are independent. Did I get something wrong? or are they really independent? because I did some research on the internet, and it told me this method should pass-by-reference (which list 'a' and 'copy' should be dependent).

public static void main(String[] args) {
    ArrayList<String> a = new ArrayList<>(Arrays.asList("X", "X"));
    ArrayList<String> copy = new ArrayList<>(a);
    copy.set(0, "B");
    copy.remove(copy.size()-1);
    System.out.println(a);
    System.out.println(copy);
}

Output:

[X, X]
[B]

Answer 1

As per the documentation, the ArrayList copy constructor:

Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.

Modifying one list has no effect on the other, which your code confirms.

Answer 2

Yes, this method should pass-by-reference (which list 'a' and 'copy' should be dependent). But these two operations don't prove this.

copy.set(0, "B");
copy.remove(copy.size()-1);

See if the following code helps you understand:

    public static void main(String[] args) {
        Process process = new Process(1);
        Process process2 = new Process(2);
        ArrayList<Process> a = new ArrayList<>(Arrays.asList(process, process2));
        ArrayList<Process> copy = new ArrayList<>(a);
        copy.get(0).id = 10;

        // This proves that both ArrayLists maintain the same Process object at this point
        // output:
        // [Id:10, Id:2]
        // [Id:10, Id:2]
        System.out.println(a);
        System.out.println(copy);


        // copy.remove(copy.size() - 1) or copy.set(0, process3) doesn't affect another ArrayList
        Process process3 = new Process(3);
        process3.id = 100;
        copy.set(0, process3);
        copy.remove(copy.size() - 1);
        // output:
        // [Id:10, Id:2]
        // [Id:100]
        System.out.println(a);
        System.out.println(copy);

    }

    static class Process {
        public int id;

        public Process(int id) {
            this.id = id;
        }

        @Override
        public String toString() {
            return "Id:" + id;
        }
    }