add two dimensional list to multidimensional arraylist

Question

I have got a loop with 2d list where I delete and add elements and want to add those temporary arraylists to the dimensions of another list.

Example,

  // 2d list
  List<List<Integer>> p = new ArrayList<List<Integer>>();
  // 3d list
  List<List<List<List<Integer>>>> list1 = new ArrayList<List<List<List<Integer>>>>();
  // this compiles ok
  list1.get(0).add(p);

but I get the following error:

  java.lang.IndexOutOfBoundsException: Index: 0, Size: 0

Generally, if you're creating a List<List<List<?>>>, it's time to create some getter / setter classes that each encapsulate a List<?>. — Gilbert Le Blanc
– Gilbert Le Blanc, Commented Jun 21, 2014 at 14:15

Ed Morales · Accepted Answer · 2014-06-21 10:26:58Z

1

You need to instantiate every list.. not just the one you are trying to access. Meaning that if you have a 2x2 matrix, you need 2 rows = 2 lists, and another list to hold them both, and so on if the matrix starts getting more complex.

List<List<Integer>> matrix = new ArrayList<List<Integer>>();
List<Integer> row = new ArrayList<Integer>();
matrix.add(row);

answered Jun 21, 2014 at 10:26

Ed Morales

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Comments

Martin · Accepted Answer · 2014-06-21 10:26:31Z

0

Your "list1" object is actually 4d in your example, but those are not initialized, so when you ask "list1" to get the first (three dimensional) list inside, it does not exist at all (arrays in Java starts at 0 size), so there is nothing to add there.

You could do something like:

List<List<List<List<Integer>>>> list1 = new ArrayList<List<List<List<Integer>>>>();

List<List<List<Integer>>>> 3dlist = new ArrayList<List<List<Integer>>>();

list1.add(3dlist)
list1.get(0).add(p);

Now, using multidimensionnal arrays directly like this is not really practical - you may want to encapsulate them in an object.

answered Jun 21, 2014 at 10:26

Martin

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3 Comments

user3758223 Over a year ago

Hi, Thanks. This works. However, when I add for example p = [1,2,3],[5,5,5] to list1.get(0).add(p); and then when I change p to equal something else like p = [0,0,0],[7,7,7] and then again I add to the other dimension list1.get(1).add(p); and then print System.out.print(list1.get(0)) it does't print just [1,2,3],[5,5,5] but everything [1,2,3],[5,5,5],[0,0,0],[7,7,7] ? The same is when I print (list1.get(1)). I want to somehow differentiate the temporary 2d lists into the dimensions to access them separately..

Martin Over a year ago

This is more and more a design question - StackOverflow is not exactly the place for that. I'm not really sure at what you try to achieve here. If you want general help on your design, you may want to try reddit.com/r/learnprogramming or something like that.

user3758223 Over a year ago

Well. What I want to achieve is quite clear I think. I want an array or any other container where I can store 2d lists. So when I call Container.get(0) it will return all 2d lists I have stored there like [1,2],[3,4] and when I call Container.get(1) it will show me another set of 2d lists like [5,6],[7,8] and so on..

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