How to use the list.toArray() method in Java for a two-dimensional array

Question

I created a list of int arrays, and I want to return it as a 2D array.

List<int[]> ans = new ArrayList<>();
int[][] toReturn = new int[ans.size()][];
return ans.toArray(toReturn);

How does this code work? What is the difference between list.toArray() and list.toArray(T[] a)?

Did you try the docs?

shmosel
– shmosel

2021-12-20 22:55:34 +00:00
Commented Dec 20, 2021 at 22:55 — shmosel
– shmosel, Commented Dec 20, 2021 at 22:55
A List<List<Integer>> would work too, by the way

OneCricketeer
– OneCricketeer

2021-12-20 23:00:23 +00:00
Commented Dec 20, 2021 at 23:00 — OneCricketeer
– OneCricketeer, Commented Dec 20, 2021 at 23:00

whiplash · Accepted Answer · 2021-12-21 00:25:54Z

1

toArray creates a brand new array while toArray(T[] arr) tries to put all elements of the list into the provided arr array.

edited Dec 21, 2021 at 0:25

whiplash

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answered Dec 20, 2021 at 22:53

Antoniossss

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1 Comment

shmosel Over a year ago

And toArray() returns an Object array instead of T[].

Yasser CHENIK · Accepted Answer · 2021-12-20 23:02:40Z

1

you just need to use foreach loop for you list and a normal loop for the array.

public int[][] toArray(ArrayList<> inputList){
int[][] toReturn = new int[inputList.size()][];
int i =0;
for(int[] arrayOfInts : inputList){
  for(int j=0;j<arrayOfInts.length;j++){
  toReturn[i][j]=arrayOfInts [j] ;
  }
i++;
}
return toReturn  ;
}

answered Dec 20, 2021 at 23:02

Yasser CHENIK

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