CODEWITHSUNDEEP
javaandroid
Max Usanin
Max Usanin

Reputation: 2499

Why can't I create a 2D ArrayList?

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>?

Upvotes: 0

Views: 137

Answers (3)

Rohit Jain
Rohit Jain

Reputation: 213213

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.

Upvotes: 2

yem yem yen
yem yem yen

Reputation: 319

A generic array of Array lists = an array list of array lists

And that would be

ArrayList<ArrayList> list = new ArrayList<ArrayList>();

Upvotes: 0

Amit Deshpande
Amit Deshpande

Reputation: 19185

You cannot create an array of a generic type. So You will need to use cast.

List<RssItem>[][] arr = (List<RssItem>[][]) new ArrayList<?>[2][2];

But still it will be good to implement List of List ArrayList<ArrayList<Type>>

List<ArrayList<RssItem>> list = new  ArrayList<ArrayList<RssItem>>();
arr[0][0] = new ArrayList<Integer>();
arr[0][0].add(10);

Upvotes: 2

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