Adding elements to cascade ArrayList in Java

Question

I have the following List to keep 3x3 matrix values:

1 2 3 
4 5 6
7 8 9

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

I want to add matrix values as shown below:

list.add(0, new ArrayList<Integer>{1, 2, 3});
list.add(1, new ArrayList<Integer>{4, 5, 6});
list.add(2, new ArrayList<Integer>{7, 8, 9});

But encounter syntax error. So, is it the correct way, or what is the proper way to assign matrix values by using cascade list?

Delete "0, ", "1, ", "2, " and it works. Or you want to use Dictionary> instead? Or was it not Dictionary in Java, something else? — BitLauncher, Jul 05 '20 at 13:12
@BitLauncher I tried before asking the question, but did not worked. Do you mean `list.add(new ArrayList{1,2,3});`? I used like that. Any idea? — , Jul 05 '20 at 13:15
@Sid You right, but it is a question and I have to use List :( Any idea? — , Jul 05 '20 at 13:15

score 2 · Answer 1 · answered Jul 05 '20 at 13:18

There is no short form syntax to create an ArrayList and add items to it such as new ArrayList<Integer>{1, 2, 3}.

If you don't need to ArrayList, I would recommend using the immutable lists created by the List.of method:

list.add(0, List.of(1, 2, 3));
list.add(1, List.of(4, 5, 6));
list.add(2, List.of(7, 8, 9));

If you must use ArrayList for some reason, do it like this:

List<Integer> arrayList0 = new ArrayList<>();
arrayList0.add(1);
arrayList0.add(2);
arrayList0.add(3);

list.add(0, arrayList0);

// repeat for index 1, 2

Maybe better to mention `List.of` is available from Java 9 – Eklavya Jul 05 '20 at 13:28 — Eklavya, Jul 05 '20 at 13:28

Arvind Kumar Avinash · Accepted Answer · 2020-07-05T13:41:08.260

You have the following options:

Java-9 onwards:

List<List<Integer>> list = new ArrayList<>();
list.add(List.of(1, 2, 3));
list.add(List.of(4, 5, 6));
list.add(List.of(7, 8, 9));

Before Java-9:

List<List<Integer>> list = new ArrayList<>();
list.add(Arrays.asList(1, 2, 3));
list.add(Arrays.asList(4, 5, 6));
list.add(Arrays.asList(7, 8, 9));

List of Integer[]

    List<Integer[]> list = new ArrayList<>();
    list.add(new Integer[] { 1, 2, 3 });
    list.add(new Integer[] { 4, 5, 6 });
    list.add(new Integer[] { 7, 8, 9 });

Using Stream API:

     List<List<Integer>> list = Stream.of(
                                         List.of(1, 2, 3), 
                                         List.of(4, 5, 6), 
                                         List.of(7, 8, 9))
                                     .collect(Collectors.toList());

Nice answer. On the last one, you might want to do a `map(ArrayList::new)` since `List.of` is immutable. The OP may want to manipulate the values. — WJS, Jul 05 '20 at 14:32

score 1 · Answer 3 · 2020-07-05T13:51:16.497

You cannot directly specify the element to add in the ArrayList constructor. Instead, you can do this.

List<List<Integer>> list = new ArrayList<>();
list.add(0, List.of(1, 2, 3));
list.add(1, List.of(4, 5, 6));
list.add(2, List.of(7, 8, 9));

Or

List<List<Integer>> list = new ArrayList<>();
list.add(0, Arrays.asList(1, 2, 3));
list.add(1, Arrays.asList(4, 5, 6));
list.add(2, Arrays.asList(7, 8, 9));

Note that List.of returns an immutable list. And Arrays.asList returns a list that can be set but not add.

score 0 · Answer 4 · answered Jul 05 '20 at 13:20

Option 1:

    list.add(new ArrayList<>(Arrays.asList(1, 2, 3)));
    list.add(new ArrayList<>(Arrays.asList(4, 5, 6)));
    list.add(new ArrayList<>(Arrays.asList(7, 8, 9)));

Option 2:

    List<List<Integer>> list = new ArrayList<>();
    List<Integer> subList;
    int n = 1;

    for(int i = 0; i < 3; ++i){
        subList = new ArrayList<>();
        for (int j = 0; j < 3; ++j){
            subList.add(n);
            n++;
        }
        list.add(subList);
    }

Welcome :) glad that it was helpful – Sercan Jul 05 '20 at 13:55 — Sercan, Jul 05 '20 at 13:55

score 0 · Answer 5 · answered Jul 05 '20 at 13:22

You can't initialize a java ArrayList like that. You can do it in the following ways:

use Arrays.asList

new ArrayList<>(Arrays.asList(1, 2, 3));
if you use guava

Lists.newArrayList(1, 2, 3)
if you use Java8

Stream.of(1, 2, 3).collect(Collectors.toCollection(ArrayList::new))

score 0 · Answer 6 · answered Jul 05 '20 at 13:38

That's another solution, a bit complex but it works:

    List<List<Integer>> list = new ArrayList<>();
    ArrayList<Integer>row1 = new ArrayList();
    ArrayList<Integer>row2 = new ArrayList();
    ArrayList<Integer>row3 = new ArrayList();
    row1.addAll(List.of(1, 2, 3));
    row2.addAll(List.of(4, 5, 6));
    row3.addAll(List.of(7, 8, 9));
    
    list.add(0,row1);
    list.add(1,row2);
    list.add(2,row3);

    System.out.println(list);

Adding elements to cascade ArrayList in Java

6 Answers6