Are Reference types in Java assigned by reference or value?

Question

As far as I am aware, in Java reference types are assigned by value, but I can't understand why the following code does what it does. Consider the following code:

Integer num = new Integer(11);
List<Integer> arr = new ArrayList<>();
arr.add(num);
List arr2 = arr;
arr = null;
System.out.println(arr2);

We create an ArrayList with one element in it, then we create the second list and assign it to the first list. My understanding that the second list just points to the same memory location as the first list, so any changes I make to the first list will apply to the second list. If I set the first list to null the second list should become null as well. Why does the console still log 11 and not null?

I believe when you are doing `arr = null` you are setting the pointer `arr` to null, not the `ArrayList` it points to. — Chris Gilardi, Sep 20 '19 at 21:26
Because you set a reference to null--and that's all. You didn't do anything to the second reference. — Dave Newton, Sep 20 '19 at 21:49
Like @mentallurg suggested It is easier to understand if you compare Java variables not to references in C, but to pointers. — Yoshikage Kira, Sep 20 '19 at 22:01

score 0 · Accepted Answer · answered Sep 20 '19 at 21:54

This is what is happening in the background. I'll try to visualize it so it is easy to understand.

Creating Integer Object:

Integer num = new Integer(11);
num --> @num493857 (some reference not real of course)
@num493857 --> ------
               | 11 |
               ------

Creating ArrayList:

List<Integer> arr = new ArrayList<>();
arr --> @arr7686
@arr7686 --> [] (empty list)

Adding num to Arraylist:

arr.add(num);
arr --> @arr7686 --> [@num493857] // @num493857 has 11 in it
                         ^
                         |
                        num

Setting arr2 = arr:

List arr2 = arr;
arr --> @arr7686 --> [@num493857]
           ^             ^
           |             |
          arr2          num

Setting arr = null:

arr = null
arr --> null
        @arr7686 --> [@num493857]
           ^             ^
           |             |
          arr2          num

Printing arr2:

System.out.println(arr2);
This prints whatever is in the list that is 11.

You don't address the author's question about references. He means C references. — mentallurg, Sep 20 '19 at 22:43
@mentallurg What do you mean? Please quote author where he is asking about C references. — Yoshikage Kira, Sep 20 '19 at 23:16

score -1 · Answer 2 · answered Sep 20 '19 at 21:52

-1

It is easier to understand if you compare Java variables not to references in C, but to pointers.

answered Sep 20 '19 at 21:52

mentallurg

4,967
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I have a feeling that this should be a comment. – Yoshikage Kira Sep 20 '19 at 22:00
1) This is an answer. An answer does not have to be extremely long. My answer explains the idea why the code of the author works as it works. And for details one should read JLS and JVM specification. 2) Many articles and many authors don't distinguish reference from pointers. which is misleading in cases like this one, because our author *does* distinguish it and thinks of reference as is it usual in C. But object variables in Java are much closer to C pointers, not to C references. – mentallurg Sep 20 '19 at 22:18

Are Reference types in Java assigned by reference or value?

2 Answers2