Initial size() of new ArrayList

Question

If I have the following:

List<String> list = new ArrayList<>(Arrays.asList("a","b","c"));

and call:

System.out.println(list.size());

It will print 3 as expected.

The size() method's code is just return size; where size is a private int.

So I'm wondering where it's actually setting that size variable.

I see it's calling the constructor public ArrayList(Collection<? extends E> c), which is fine, but when I debug into there, and hover over c, it already says size = 3.

So is it something in Collection<? extends E> c that's setting it to 3, before it gets to the ArrayList constructor?

Are you talking about http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/tip/src/share/classes/java/util/ArrayList.java#l169? — Micha Wiedenmann, Oct 19 '18 at 13:33
What do you mean by `c == 3`? It's a Collection and not an int. — Murat Karagöz, Oct 19 '18 at 13:34
Wow, heavy downvotes. @MuratKaragöz when I debug to that constructor, and hover over `c`, it says `size = 3`. I thought this was a valid question. — achAmháin, Oct 19 '18 at 13:40
*Wow, heavy downvotes* - also heavy upvotes. I would be content with a balance of 0 for a question you can answer simply by looking at the source — Tim, Oct 19 '18 at 13:43
@TimCastelijns fair enough. I did look at the source code, but unfortunately I mis-read that specific line. But it answers my question. — achAmháin, Oct 19 '18 at 13:47
@achAmháin maybe some IDE feature confused you? Something like the image I posted in my answer. It has happened to my from time to time ;) — lealceldeiro, Oct 19 '18 at 14:00

lealceldeiro · Answer 1 · 2018-10-19T14:15:00.040

(...) I'm wondering where it's actually setting that size variable

You called this constructor of ArrayList in which source code we can see size = elementData.length;:

public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    size = elementData.length; // <-- HERE
    if (elementData.getClass() != Object[].class)
        elementData = Arrays.copyOf(elementData, size, Object[].class);
}

So, once it is called size() (source code) that value is already set.

when I debug into there, and hover over c, it already says size = 3

If you are referring to something like the image below... I think it might be some IDE feature (like in my case with IntelliJ IDEA) where it can infer the specific type of c (List) and compute its size.

(...) is it something in Collection<? extends E> c that's setting it to 3, before it gets to the ArrayList constructor?

Remember Arrays.asList returns a List which is taken by the ArrayList's constructor as a Collection which also has a size method. IDEs can use that for calculating the size value.

PS: The source code might slightly differ from specific jdk version to another version.

score 1 · Answer 2 · answered Oct 19 '18 at 13:34

It says it's already 3 because that is the size of the Arrays.asList("a","b","c") you pass as argument to new ArrayList<>(Arrays.asList("a","b","c"));.

From sources:

public ArrayList(Collection<? extends E> c) {
    // c is already instantiated here 
    elementData = c.toArray();
    // ...
}

score 1 · Accepted Answer · answered Oct 19 '18 at 13:36

It happens when you initialise the new ArrayList; in the constructor, you can see the size of the list is set to the size of the array you pass in:

/**
 * Constructs a list containing the elements of the specified
 * collection, in the order they are returned by the collection's
 * iterator.
 *
 * @param c the collection whose elements are to be placed into this list
 * @throws NullPointerException if the specified collection is null
 */
public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    if ((size = elementData.length) != 0) {
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    } else {
        // replace with empty array.
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

so specifically if ((size = elementData.length) != 0) sets the size (the private field you were talking about) to the length of the array passed into the constructor.

Thanks. Somehow I misread that line, so started looking for where it sets `size` elsewhere. — achAmháin, Oct 19 '18 at 13:52

score 0 · Answer 4 · answered Oct 19 '18 at 13:39

0

Here, the size is set on the next line in the constructor of the ArrayList.

if ((size = elementData.length) != 0) {

Details are well explained in this other SO thread Details

answered Oct 19 '18 at 13:39

keybored

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Initial size() of new ArrayList

4 Answers4