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?

• Mick Mnemonic: Would you want the data structure to reflect the main category -> sub category hierarchy? Perhaps a Map<String, List<String>> (mapping main categories into lists of sub categories) would be a better choice then.
• alcfeoh: I agree with Mic, a map is probably a better option in that case.
• Kick Buttowski: @MickMnemonic why map? because of collision problem and find any arraylist type string faster?
• Max Echendu: @MickMnemonic I was trying to avoid using Map or HashMaps. But I suppose that's just me being lazy.
• Mick Mnemonic: @KickButtowski, no, because a map is ideal for building hierarchical (tree-like) models, ones like the OP's example seems to represent.

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

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

Map<String, List<String>> mainCategories = new TreeMap<>();
mainCategories.put("A", Arrays.asList("A1", "A2", "A3"));
mainCategories.put("B", Arrays.asList("B1", "B2"));
mainCategories.put("C", Arrays.asList("C1"));
System.out.println(mainCategories);
System.out.println(mainCategories.get("B"));

This prints out

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

Answer 3

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.

• Kick Buttowski: your list should have a type don't u think? it feels very raw ?
• Vince: @KickButtowski What are you talking about? His first list uses List as a type argument, the second one uses String. Both have type arguments specified
• Kick Buttowski: @VinceEmigh he eidted his answer

Answer 4

Sure you can.

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

• Kick Buttowski: if you can add good explanation, I am more than happy to vote you up