Can I make an ArrayList in an ArrayList (java)?

Question

I' would like to store user input into an ArrayList then place that ArrayList into another ArrayList. So sort of like a main category, which contains sub-categories that contain data.

Here is some of the code:

    ArrayList<String> mainCat = new ArrayList<>();
    ArrayList<String> subCat = new ArrayList<>();

Could I add the "subCat" ArrayList in the "mainCat" ArrayList?

Answer 1

Of course! Add it just like any other ArrayList, except the type would be ArrayList<String>.

ArrayList<ArrayList<String>> mainCat = new ArrayList<ArrayList<String>>();
ArrayList<String> subCat = new ArrayList<>();
mainCat.add(subCat);

Answer 2

why not ? But you need to make a slight change in the definition of mainCat:

List<List<String>> mainCat = new ArrayList<List<String>>();

Because items in mainCat collection will not be String but collections.

Answer 3

I would use a RewriteRule for this purpose, because it makes it easier to lookup sub categories based on the main category. If you use a Map, you actually have no way to determine into which main category a given sub category belongs to. Here's an example using a List<List<String>> (which automatically keeps main categories in alphabetical order):

TreeMap

This prints out

{A=[A1, A2, A3], B=[B1, B2], C=[C1]}
[B1, B2]

Answer 4

Sure you can.

List<List<String>> mainCat = new ArrayList<ArrayList<String>>();
List<String> subCat = new ArrayList<String>();

mainCat.add(subCat);