Initialize ArrayList<ArrayList<Integer>>

Question

I met a problem as follow:

When I initialize a ArrayList<ArrayList<Integer>>, the codes are:

ArrayList<ArrayList<Integer>> group = new ArrayList<ArrayList<Integer>>();
group.add((ArrayList<Integer>) Arrays.asList(1, 2, 3));
group.add((ArrayList<Integer>) Arrays.asList(4, 5, 6));
group.add((ArrayList<Integer>) Arrays.asList(7, 8, 9));
for (ArrayList<Integer> list : group) {
    for (Integer i : list) {
        System.out.print(i+" ");
    }
    System.out.println();
}

Although the codes can be compiled successfully, I still get a exception on console:

Exception in thread "main" java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to java.util.ArrayList at Solution.main(Solution.java:49)

Thanks for help !

This strikes me as something that should be caught at compilation time. What's with the forced cast? — Blindy
– Blindy, Commented Nov 24, 2014 at 19:56

Jon Skeet · Accepted Answer · 2014-11-24 20:04:43Z

12

Arrays.asList doesn't return a java.util.ArrayList. It does return an instance of a class called ArrayList, coincidentally - but that's not java.util.ArrayList.

Unless you need this to really be an ArrayList<ArrayList<Integer>> I'd just change it to:

List<List<Integer>> group = new ArrayList<>();
group.add(Arrays.asList(1, 2, 3));
group.add(Arrays.asList(4, 5, 6));
group.add(Arrays.asList(7, 8, 9));
for (List<Integer> list : group) {
    ...
}

If you do need an ArrayList<ArrayList<...>> - or if you need to be able to add to the "inner" lists even if you don't need them with a static type of ArrayList - then you'll need to create a new ArrayList for each list:

group.add(new ArrayList<Integer>(Arrays.asList(1, 2, 3)));
// etc

edited Nov 24, 2014 at 20:04

answered Nov 24, 2014 at 19:52

Jon Skeet

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1 Comment

Jon Skeet Over a year ago

@manouti: Doh! I copied that line specifically to do that... Done.

rgettman · Accepted Answer · 2014-11-24 19:51:46Z

5

The return of Arrays.asList is not a java.util.ArrayList; the java.util.Arrays$ArrayList is a separate class, nested in Arrays, even if it's a List.

If you must have an ArrayList, then create another ArrayList yourself using the returned List from Arrays.asList, instead of casting, e.g.:

group.add(new ArrayList<Integer>( Arrays.asList(1, 2, 3) ));

answered Nov 24, 2014 at 19:51

rgettman

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Comments

M A · Accepted Answer · 2014-11-24 19:53:08Z

4

Arrays.asList returns an instance of a nested static type java.util.Arrays.ArrayList that is different from java.util.ArrayList. You can avoid this problem by programming to the List interface (which java.util.Arrays.ArrayList implements as well) and without the unneeded casts:

List<List<Integer>> group = new ArrayList<List<Integer>>();
group.add(Arrays.asList(1, 2, 3));
group.add(Arrays.asList(4, 5, 6));
group.add(Arrays.asList(7, 8, 9));
for (List<Integer> list : group) {
    for (Integer i : list) {
        System.out.print(i+" ");
    }
    System.out.println();
}

answered Nov 24, 2014 at 19:53

M A

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Chandrasheker Shivvolla · Accepted Answer · 2017-08-17 07:59:20Z

1

group.add(new ArrayList<Integer>((Arrays.asList(1, 2, 3))));
group.add(new ArrayList<Integer>((Arrays.asList(4, 5, 6))));
group.add(new ArrayList<Integer>((Arrays.asList(7, 8, 9))));

for (ArrayList<Integer> arrayList : group) {
  for (Integer integer : arrayList) {
    System.out.println(integer);
  }
}

Please refer this code. This might help you out to achieve your requirement.

edited Aug 17, 2017 at 7:59

user8317956

answered Aug 17, 2017 at 6:44

Chandrasheker Shivvolla

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1 Comment

yakobom Over a year ago

How does your answer really differ from the already accepted one?

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