Question

I am trying to write the following code…

static ArrayList<RssItem>[][] list = new ArrayList<RssItem>[][]; 

This doesn’t work. What am I doing wrong, if I am trying to create a list of ArrayList<RssItem>?

Answer 1

You need to give the size on the RHS: –

static List<RssItem>[][] arr = (List<RssItem>[][])new List<?>[4][];

Size of column is not needed, but you can give it too: –

static List<RssItem>[][] arr = (List<RssItem>[][])new List<?>[4][5];

NOTE: –

You cannot give a specific type as a type parameter on the RHS. You would have to use WildCard. Specific type would only be given when you are instantiating your list in the array.

To add something to your array, you can use this kind of loop: –

NOT Tested

    List<RssItem>[][] arr = (List<RssItem>[][])new ArrayList<?>[4][4];

    for (int i = 0; i < arr.length; i++) {
        for (int j = 0; j < arr[i].length; j++) {
            arr[i][j] = new ArrayList<RssItem>();

            arr[i][j].add(new RssItem());
        }
    }

    for (List<RssItem>[] val: arr) {
       for (List<RssItem> val2: val) {
           System.out.println(val2);
       }
    }

P.S.: –

Please don’t use 2-D array of List. That is equivalent to a 3-D array. Even if you get the above loop to work somehow. It will make your code mess in future. And your life hell. There are many other data structures in Java, that you should consider, like a Map, which is useful in such kind of situations.

Even, having a List upto 3 levels is a very bad idea. So, please change your design.