I wanted to initialize a 2D arraylist of size 2n-1(rows), n(cols)
int num = 3;
// declare an arrayList of ArrayLists or 2D ArrayList
ArrayList<ArrayList<Integer>> list =
new ArrayList<ArrayList<Integer>>(num);
The above syntax is to create 2D ArrayList of size Num*Num.
In arrays we initialize with M*N, similarly can we do with ArrayLists? if yes, how?
I want syntax to initialize with M*N.
Unlike an Array which must be initialized to a fixed length, an ArrayList can grow dynamically, you don't have to worry about M and N to initialize it. You worry about that when you want to fill the ArrayList and if filling the collection to M and N is your specific requirement then do so after the ArrayList has been declared. So, ArrayList<ArrayList<Integer>> list = new ArrayList<>(); is all you need.
There are a number of ways to fill the ArrayLists to your desired M (rows) and N (columns). Here is a relatively decent visual way to do it.
ArrayList<ArrayList<Integer>> list = new ArrayList<>();
ArrayList<Integer> innerList;
int m = 20; // Rows
int n = 10; // Columns
int incrementer = 1;
for (int i = 0; i < m; i++) {
innerList = new ArrayList<>();
for (int j = 0; j < n; j++) {
innerList.add(j + incrementer);
}
list.add(innerList);
incrementer ++;
}
// Now to display (print) the 2D ArrayList into the Console Window:
for (ArrayList<Integer> inner : list) {
for (Integer ints : inner) {
System.out.printf("%-3s ", ints);
}
System.out.println();
}
The console should display:
1 2 3 4 5 6 7 8 9 10
2 3 4 5 6 7 8 9 10 11
3 4 5 6 7 8 9 10 11 12
4 5 6 7 8 9 10 11 12 13
5 6 7 8 9 10 11 12 13 14
6 7 8 9 10 11 12 13 14 15
7 8 9 10 11 12 13 14 15 16
8 9 10 11 12 13 14 15 16 17
9 10 11 12 13 14 15 16 17 18
10 11 12 13 14 15 16 17 18 19
11 12 13 14 15 16 17 18 19 20
12 13 14 15 16 17 18 19 20 21
13 14 15 16 17 18 19 20 21 22
14 15 16 17 18 19 20 21 22 23
15 16 17 18 19 20 21 22 23 24
16 17 18 19 20 21 22 23 24 25
17 18 19 20 21 22 23 24 25 26
18 19 20 21 22 23 24 25 26 27
19 20 21 22 23 24 25 26 27 28
20 21 22 23 24 25 26 27 28 29
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