Fastest way to get the first n elements of a List into an Array

Option 1 Faster Than Option 2

Because Option 2 creates a new List reference, and then creates an n element array from the List (option 1 perfectly sizes the output array). However, first you need to fix the off by one bug. Use < (not <=). Like,

String[] out = new String[n];
for(int i = 0; i < n; i++) {
    out[i] = in.get(i);
}

Option 3

Iterators are faster than using the get operation, since the get operation has to start from the beginning if it has to do some traversal. It probably wouldn't make a difference in an ArrayList, but other data structures could see a noticeable speed difference. This is also compatible with things that aren't lists, like sets.

String[] out = new String[n];
Iterator<String> iterator = in.iterator();
for (int i = 0; i < n && iterator.hasNext(); i++)
    out[i] = iterator.next();

Assumption:

list - List<String>

Using Java 8 Streams,

• to get first N elements from a list into a list,

  List<String> firstNElementsList = list.stream().limit(n).collect(Collectors.toList());

• to get first N elements from a list into an Array,

  String[] firstNElementsArray = list.stream().limit(n).collect(Collectors.toList()).toArray(new String[n]);

It mostly depends on how big n is.

If n==0, nothing beats option#1 :)

If n is very large, toArray(new String[n]) is faster.