Question

So I’ve got this code which will add the numbers 1-9 into separate ArrayLists if the ArrayList doesn’t exist. However, even though I print the ArrayLists(and it gets me all the correct numbers), when I print the .size of the ArrayList, it gives me 1 instead of 9. I hope you understand my problem. Here’s the code:

ArrayList[][] tillatnaSiffror = new ArrayList[9][9];

    for(int i=0;i<9;i++){
        for(int ruta=0;ruta<9;ruta++){

            if(tillatnaSiffror[i][ruta] == null){
                for(int add=1;add<=9;add++){
                    tillatnaSiffror[i][ruta] = new ArrayList<Integer>();
                    tillatnaSiffror[i][ruta].add(add);
                    System.out.println(tillatnaSiffror[i][ruta]);
                }
                System.out.println(tillatnaSiffror[i][ruta].size());
            }
        }
    }

That gives me this(although nine times of course): [1][2][3][4][5][6][7][8][9]1

Now I’m wondering, WHY do I get 1 instead of 9 when I print .size?

Answer 1

Because you reset the list in each iteration by doing

tillatnaSiffror[i][ruta] = new ArrayList<Integer>();

i.e., you create a new list, throwing away the previous one for each digit you add!

Try moving out the creation of the list:

 ,-->   tillatnaSiffror[i][ruta] = new ArrayList<Integer>();
 |      for(int add=1;add<=9;add++){
 '-----<
            tillatnaSiffror[i][ruta].add(add);
            System.out.println(tillatnaSiffror[i][ruta]);
         }

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As a side note, I would suggest to avoid arrays here, and use Java collections all the way. Consider for instance to use a structure like List<List<Set<Integer>>>.