[FIXED] how to get list<item> from list<list<item>>

Issue

I want to get list from list of lists like

List<List<Item>> items = new ArrayList<List<Item>>(4);   
List<Item> item = items.get(1);

This it not working.
I want a type ArrayList<Item>.

Solution

When you use the ArrayList(int) constructor, it creates an empty list. It cannot fill the list with instances of the given type, it fills it with null.

Another remark: items.get(1) would give you the second element in the list, because indices in Java start at 0. The first element would be accessed by items.get(0).

Here is how you could fill the list with empty lists:

List<List<Item>> items = new ArrayList<>();
for (int i=0; i < 4; i++) {
    items.add(new ArrayList<>());
}
List<Item> item = items.get(0); // an empty ArrayList

You can also make the ArrayList unmodifiable by wrapping it with Collections.unmodifiableList(List).

Or even better, only make a new list when needed.

private List<List<Item>> items = new ArrayList<>(4);
public List<Item> getItem(int index) {
    List<Item> ret = items.get(index);
    if (ret == null) {
        ret = new ArrayList<>();
        items.set(index, ret);
    }
    return ret;
}

However, if you don't want to change the size of the list, may I suggest just a normal array?

Item[][] items = new Item[4][];
// OR
List<Item>[] items = new List<Item>[4]; // however this combination is pretty weird

Answered By - Wasabi Thumbs
Answer Checked By - Clifford M. (JavaFixing Volunteer)