Java 2D array into 2D ArrayList with stream

Question

So I have an Integer[][] data that I want to convert into an ArrayList<ArrayList<Integer>>, so I tried using streams and came up with the following line:

ArrayList<ArrayList<Integer>> col = Arrays.stream(data).map(i -> Arrays.stream(i).collect(Collectors.toList())).collect(Collectors.toCollection(ArrayList<ArrayList<Integer>>::new));

But the last part collect(Collectors.toCollection(ArrayList<ArrayList<Integer>>::new)) gives me an error that it cannot convert ArrayList<ArrayList<Integer>> to C.

If you don't need a specific List implementation, you can do List<List<Integer>> col = Arrays.stream(data).map(Arrays::asList).collect(Collectors.toList()); — Alexis C.
– Alexis C., Commented May 3, 2016 at 9:19

Eran · Accepted Answer · 2016-05-03 09:09:56Z

4

The inner collect(Collectors.toList() returns a List<Integer>, not ArrayList<Integer>, so you should collect these inner Lists into an ArrayList<List<Integer>> :

ArrayList<List<Integer>> col = 
    Arrays.stream(data)
          .map(i -> Arrays.stream(i)
                          .collect(Collectors.toList()))
          .collect(Collectors.toCollection(ArrayList<List<Integer>>::new));

Alternately, use Collectors.toCollection(ArrayList<Integer>::new) to collect the elements of the inner Stream :

ArrayList<ArrayList<Integer>> col = 
     Arrays.stream(data)
           .map(i -> Arrays.stream(i)
                           .collect(Collectors.toCollection(ArrayList<Integer>::new)))
           .collect(Collectors.toCollection(ArrayList<ArrayList<Integer>>::new));

edited May 3, 2016 at 9:09

answered May 3, 2016 at 9:07

Eran

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2 Comments

seMikel Over a year ago

Ah, I see. I thought it would cast List to ArrayList on it's own. Thanks!

Holger Over a year ago

You don’t need to write such prose at constructor references. Thanks to type inference, writing ArrayList::new is enough. And I wouldn’t use a stream operation for the inner array conversion, .map(sub -> new ArrayList<>(Arrays.asList(sub))) is much simpler…

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