How to convert List to int[] in Java? [duplicate]

Answer 1

Using a lambda you could do this (compiles in jdk lambda):

public static void main(String ars[]) {
        TransformService transformService = (inputs) -> {
            int[] ints = new int[inputs.size()];
            int i = 0;
            for (Integer element : inputs) {
                ints[ i++ ] = element;
            }
            return ints;
        };

        List<Integer> inputs = new ArrayList<Integer>(5) { {add(10); add(10);} };

        int[] results = transformService.transform(inputs);
    }

    public interface TransformService {
        int[] transform(List<Integer> inputs);
    }

Answer 2

This simple loop is always correct! no bugs

  int[] integers = new int[myList.size()];
  for (int i = 0; i < integers.length; i++) {
      integers[i] = myList.get(i);
  }

Answer 3

In addition to Commons Lang, you can do this with Guava's method Ints.toArray(Collection<Integer> collection):

List<Integer> list = ...
int[] ints = Ints.toArray(list);

This saves you having to do the intermediate array conversion that the Commons Lang equivalent requires yourself.

Answer 4

int[] ret = new int[list.size()];       
Iterator<Integer> iter = list.iterator();
for (int i=0; iter.hasNext(); i++) {       
    ret[i] = iter.next();                
}                                        
return ret;                              

Answer 5

Java 8 has given us an easy way to do this via streams...

Using the collections stream() function and then mapping to ints, you'll get an IntStream. With the IntStream we can call toArray() which gives us int []

int [] ints = list.stream().mapToInt(Integer::intValue).toArray();

to int []

to IntStream

Answer 6

try also Dollar (check this revision):

import static com.humaorie.dollar.Dollar.*
...

List<Integer> source = ...;
int[] ints = $(source).convert().toIntArray();