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
– Robin-Hoodie, Commented May 27, 2015 at 11:22
Performance wise the second method will be slower you are pre-allocating memory — The6thSense
– The6thSense, Commented May 27, 2015 at 11:23
@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
– JB Nizet, Commented May 27, 2015 at 11:31
@TheLostMind Arrays#copyOf is intrinsic in hotspot so that code is not really run (so is System.arraycopy) - see line 748 of this file. — assylias
– assylias, Commented May 27, 2015 at 11:56

TheLostMind · Accepted Answer · 2015-05-27 11:33:16Z

11

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

edited May 27, 2015 at 11:33

TheLostMind

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answered May 27, 2015 at 11:25

Ouney

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2 Comments

Vimal Bera Over a year ago

@TheLostMind - Now, it's pretty clear from the code snippet...!! Thanks

Ouney Over a year ago

additional 1 for improving my answer :)

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