Difference between converting List to Array

Question

I was just wondering what is the difference between following two approach of converting List to Array.

List<String> test = new ArrayList<String>();
test.add("AB");
test.add("BC");
test.add("CD");
test.add("DE");
test.add("EF");

String[] testarray = test.toArray(new String[0]); // passing 0 as Array size

And below one :

List<String> test = new ArrayList<String>();
test.add("AB");
test.add("BC");
test.add("CD");
test.add("DE");
test.add("EF");

String[] testarray = test.toArray(new String[test.size()]); // passing list's size

I got the same output for testarray on console.

This is what the docs for JDK7 have to say about this: **toArray(T[] a) Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.** — Robin-Hoodie, May 27 '15 at 11:22
Performance wise the second method will be slower you are pre-allocating memory — The6thSense, May 27 '15 at 11:23
Link to documentation: https://docs.oracle.com/javase/7/docs/api/java/util/List.html#toArray() — User404, May 27 '15 at 11:23
@NiklasR Also from documentation: ' If the list fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this list.' — User404, May 27 '15 at 11:24
Duplicate of http://stackoverflow.com/questions/14280631/pass-zero-sized-array-save-allocation — scottb, May 27 '15 at 11:26
@VigneshKalai the first one will actually be marginally slower, since you create a useless empty array and force toList() to create a second one of the right size. — JB Nizet, May 27 '15 at 11:31
@JBNizet now it's make more sense in terms of performance..!! — Vimal Bera, May 27 '15 at 11:33
@JBNizet - If the size of the array being passed is sufficiently large (to hold elements of list), then the first one will be *marginally faster* :P — TheLostMind, May 27 '15 at 11:35
@JBNizet That's what I thought too, until I tested it: http://stackoverflow.com/a/29444594/829571 — assylias, May 27 '15 at 11:35
@assylias I think we can all agree that if the goal is to have a new array and not to overwrite an existing array, both are equivalent and the difference in performance is so negligible that it doesn't really matter. — JB Nizet, May 27 '15 at 11:39
@assylias - The code actually suggests otherwise. If the size of array is `0`, then `Arrays.copyOf` is called. Whereas if the size of array is large enough to hold all elements of list, then `System.arraycopy` is called. Internally, `Arrays.copyOf` also calls `System.arraycopy` after executing some *rather complex* instruction(s). — TheLostMind, May 27 '15 at 11:44
@TheLostMind Arrays#copyOf is intrinsic in hotspot so that code is not really run (so is System.arraycopy) - see line 748 of [this file](http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/87ee5ee27509/src/share/vm/classfile/vmSymbols.hpp#l748). — assylias, May 27 '15 at 11:56
@assylias - Oh. So the code is optimized by the hotspot.. Thanks :) — TheLostMind, May 27 '15 at 12:07

score 11 · Accepted Answer · edited May 27 '15 at 11:33

public <T> T[] toArray(T[] a)

a - This is the array into which the elements of the list are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose. So in first case a new array is being created while in second case, it is using the same array.

Sample Code :

Case -1 : array being passed can hold elements of list

public static void main(String[] args) {
        List<String> test = new ArrayList<String>();
        test.add("AB");
        test.add("BC");
        test.add("CD");
        test.add("DE");
        test.add("EF");
        String[] s= new String[10];
        String[] testarray = test.toArray(s); 
        System.out.println(s==testarray);
    }

O/P :

true

Case-2 : Array being passed cannot hold elements of list

public static void main(String[] args) {
        List<String> test = new ArrayList<String>();
        test.add("AB");
        test.add("BC");
        test.add("CD");
        test.add("DE");
        test.add("EF");
        String[] s= new String[0];
        String[] testarray = test.toArray(s); 
        System.out.println(s==testarray);

    }

O/P :

false

@TheLostMind - Now, it's pretty clear from the code snippet...!! Thanks — Vimal Bera, May 27 '15 at 11:36

Difference between converting List to Array

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