How to convert List<Integer> to int[] in Java?

Question

This is similar to this question: http://stackoverflow.com/questions/880581/java-convert-int-to-integer

I'm new to Java. How can i convert a List to int[] in Java? I'm confused because List.toArray() actually returns an Object[], which can be cast to nether Integer[] or int[].

Right now I'm using a loop to do so:

int[] toIntArray(List<Integer> list){
  int[] ret = new int[list.size()];
  for(int i = 0;i < ret.length;i++)
    ret[i] = list.get(i);
  return ret;
}

I'm sure there's a better way to do this.

Answer 1

Unfortunately, I don't believe there really is a better way of doing this due to the nature of Java's handling of primitive types, boxing, arrays and generics. In particular:

• List<T>.toArray won't work because there's no conversion from Integer to int
• You can't use int as a type argument for generics, so it would have to be an int-specific method (or one which used reflection to do nasty trickery).

I believe there are libraries which have autogenerated versions of this kind of method for all the primitive types (i.e. there's a template which is copied for each type). It's ugly, but that's the way it is I'm afraid :(

Even though the Arrays class came out before generics arrived in Java, it would still have to include all the horrible overloads if it were introduced today (assuming you want to use primitive arrays).

Answer 2

There is really no way of "one-lining" what you are trying to do because toArray returns an Object[] and you cannot cast from Object[] to int[] or Integer[] to int[]

Answer 3

The easiest way to do this is to make use of Apache Commons Lang. It has a handy ArrayUtils class that can do what you want. Use the toPrimitive method with the overload for an array of Integers.

List<Integer> myList;
 ... assign and fill the list
int[] intArray = ArrayUtils.toPrimitive(myList.toArray(new Integer[myList.size()]));

This way you don't reinvent the wheel. Commons Lang has a great many useful things that Java left out. Above, I chose to create an Integer list of the right size. You can also use a 0-length static Integer array and let Java allocate an array of the right size:

static final Integer[] NO_INTS = new Integer[0];
   ....
int[] intArray2 = ArrayUtils.toPrimitive(myList.toArray(NO_INTS));

Answer 4

int[] toIntArray(List<Integer> list)  {
    int[] ret = new int[list.size()];
    int i = 0;
    for (Integer e : list)  
        ret[i++] = e.intValue();
    return ret;
}

Slight change to your code to avoid expensive list indexing.

Answer 5

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 6

try also Dollar (check this revision):

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

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