Fill a array with List data with one more element

Question

Hello,

By a question that I made, I figured out that tho copy elements from one list to an array I just need to use the method toArray() for this.

But let's suppose I have a List with n objects. I want to copy then into a array sized n+1 and add into the first position another object and in the other n positions the n data of the list.

This is the way I'm doing it for now, but I'm just wondering if there is a better way for do that:

    Object array[] = new Object[list.size() + 1];

    Object chk = new Object();

    array[0] = chk;

    for(int i = 1; i < array.length; i++){
        array[i] = list.get(i);
    }

Answer 1

If you're looking for a way to avoid having to loop through the array and the list, I think you're out of luck. Java doesn't provide a way to do a mass copy except for ones that just use a API call that does the looping through the collection behind the scenes, hidden from view.

Answer 2

You could used a LinkedList and use offerFirst() and then toArray(), but I doubt it really matters.

Answer 3

The only way to do this without your loop would be:

Object array[] = new Object[list.size() + 1];
Object oldValues[] = list.toArray();

Object chk = new Object();
array[0] = chk;
System.arrayCopy(oldValues, 0, array, 1, oldValues.length);

System.arrayCopy is a bit faster than looping (not too much, but faster nevertheless), although the performance boost would probably be mitigated by the toArray, depending on the implementation of your List.

All in all, what you have is a decent approach as long as List.get(int) is an acceptable cost. (If it's O(n), you'll end up with O(n^2) for your operation, which is suboptimal when Frank's solution is O(n).)

Answer 4

use an iterator:

...

int i = 1;
for(Object item:list){
    array[i] = item;
    i++;
}

Answer 5

I didn't understood what you are trying to achieve with that, but if I understood the problem correctly, this is how I would do it:

    List<Object> elementList = new ArrayList<Object>();
    Object additionalElement = new Object();
    Object array[] = null;

    //[Add code to populate the List]

    //Add the additional element
    elementList.add(0,additionalElement);

    array = elementList.toArray();

This would do the trick with the advantage of not iterating anything.