Convert from List> to List

Question

i want to convert from List<List<Integer>> to List<int[]> but i don't know how to do it.

I try to do it :

List<Integer> list = new ArrayList<>();
List<List<Integer>> arr = matrix(list);
int [] temp2 = arr;

But it was wrong

Answer 1

I am not sure what you really want to achieve, but a simple solution would be a double loop to convert each List<Integer> to int[]. And you can do it with a simple method as AFAIK there is no built-in library doing this for you (because of the heterogeneity of types List and int[]):

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

Update Java 8

Thanks to Java 8 streams now you can simply write (again the example is for a single List<Integer>:

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

Update

As you asked for the complete solution, I will write down a possible method to achieve what you need:

List<int[]> intArrayList = new ArrayList();

for(List<Integer> intList : intListList){
    int[] intArray = new int[intList.size()];
    for(int i = 0; i < intArray.length; ++i){
       intArray[i] = intList.get(i);
    }
    intArrayList.add(intArray);
}

Answer 2

 i want to convert from List<List<Integer>> to List<int[]> 

The return value from matrix() that you called arr for some reason is called intListList below.

// You can map the List<Integer> items in the List<List<Integer> to int[] items in a List<int[]> like so:

List<int[]> intArrList = intListList.stream()
    .map(Collection::stream)
    .map(Stream::mapToInt)
    .map(IntStream::toArray)
    .collect(Collectors.toList())

// or if you prefer

List<int[]> intArrList = intListList.stream()
    .map(ints -> ints.stream().mapToInt().toArray())
    .collect(Collectors.toList())

// For Java before 8
List<int[]> intArrList = new ArrayList();
for(List<Integer> ints : intListList){
    int[] intArr = new int[ints.size()];
    for(int i = 0; i < intArr.length; ++i){
       intArr[i] = ints.get(i);
    }
    intArrList.add(intArr);
}