How to convert List of Integers to int array in Java - Coding Tips

So, you have a List of Integers and you want to convert them into int
array? Yes you read it write, not on Integer
array but int array. Though in
most practical purpose, Integer array can be used in place of int[] because of
autoboxing in Java, you still
need an int[] if your method accepts it. In Java, you can
not type cast an Integer array into
int array. Many Java programmer think about toArray() method
from java.util.List to convert a List into Array, but unfortunately toArray() is useless
in most of times. It doesn't allow you to convert List into
primitive arrays. Though you can convert List of Integers to array
of Integers, but not array of primitive int. This is
true for List of all wrapper class e.g. List of Float, Double, Long, toArray() method can
return array of wrapper class but not primitives. After looking into Java
Collection API, It seems that only traditional for loop or foreach can help,
which involves iterating over Integer array
and storing them into int[], but fortunately, I came across Apache
Commons ArrayUtils class. ArrayUtils can do this
work for us, It has several overloaded methods to convert
Object arrays into primitive arrays. For example, you can convert an array of
Double i.e. Double[] to primitive double array e.g. double[]. This example
once again reinforced my thinking to include Apache Commons lang and Google's
Guava by default in any Java project, they are rich and effectively complement
standard JDK library.

List of Integer to int array in Java

Here is complete Java program to convert Integer List into int array. We have
first used toArray() method to convert List into Array
of same type, and then we use ArrayUtils to convert
wrapper class array into primitive array. By the way, if you don't like to add
dependencies on your project, you can also use simple for loop to iterate over
Integer array and storing them back into int[]. here is
the code.

import java.util.Arrays;
import java.util.List;
import org.apache.commons.lang.ArrayUtils;

/**
 * Java program to convert List of Integers into int array using Apache commons ArrayUtils class.
 *
 * @author Javin Paul
 */
public class IntegerListToIntArray {
   
    public static void main(String args[]) {
  
       List<Integer> numbers = Arrays.asList(1,2,3,4,5,6);
      
       System.out.println("List of Integers : " + numbers);
      
       // toArray() can return Integer array but not int array
       Integer[] integers = numbers.toArray(new Integer[numbers.size()]);
      
       // ArrayUtils of Apache Commons can change an Object array to primitive array
       int[] primitives = ArrayUtils.toPrimitive(integers);
      
       // Let's see what does this int array contains
       System.out.println("Array with primitive int : " + Arrays.toString(primitives));
    }     
   
}

Output
List of Integers : [1, 2, 3, 4, 5, 6]
Array with primitive int : [1, 2, 3, 4, 5, 6]

As you can see, It's pretty straight forward to convert List of Integers
to int array, though it requires more than one method call to do that. It would
have been much better, if toArray() can return primitive arrays as
well. On similar note, conversion of Arrays are bit tricky in Java as int[]
!= Integer[] , so keep that in mind while writing Java API. Prefer List over Array
for public methods, if you had to accept arrays, try to minimize scope of those
method by making them private or package-private.

Source:http://javarevisited.blogspot.com/2013/05/how-to-convert-list-of-integers-to-int-array-java-example-tips.html