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

Question

What is the fastest way to get the first n elements of a list stored in an array?

Considering this as the scenario:

int n = 10; ArrayList<String> in = new ArrayList<>(); for(int i = 0; i < (n+10); i++)   in.add("foobar"); 

Option 1:

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

Option 2:

String[] out = (String[]) (in.subList(0, n)).toArray(); 

Option 3: Is there a faster way? Maybe with Java8-streams?

Answer 1

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]);

Answer 2

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); }