Java - How to add elements on a multidimensional arraylist?

Question

I have this array: ArrayList<ArrayList<Integer>> items = new ArrayList<ArrayList<Integer>>();

I want to create this array on a for cicle and the result must be something like:

[0]: {0 , 0, 0, 3};
[1]: {1 , 2, 3, 3};
[2]: {6 , 2, 5, 4};

During the cicle, new values must be added to the ArrayList in a certain index. The indexcan contains values or not, but they must not be overwriten.

Can i add only arrays to a certain index like items.add(index, array);?

So, how to add more values without overwrite anything?

Krzysztof Cichocki · Accepted Answer · 2015-05-21 18:41:58Z

1

Best solution is to use HashMap, like this:

HashMap<Integer, int[]> items = new HashMap<Integer, int[]>();

items.put(0, new int[] {0 , 0, 0, 3});
items.put(1, new int[] {1 , 2, 3, 3});
items.put(2, new int[] {6 , 2, 5, 4});

Then you can set and get any value at any given key, even if the key does not exist yet.

You can sipmly use:

int[] row = items.get(0);

to get the desired row.

edited May 21, 2015 at 18:41

answered May 21, 2015 at 18:26

Krzysztof Cichocki

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Comments

Steven Vaccaro · Accepted Answer · 2015-05-21 19:01:50Z

If I understand correctly, you are trying to add whole integer array(s) to an ArrayList>. In this case you might try something like:

    ArrayList<ArrayList<Integer>> items = new ArrayList<ArrayList<Integer>>();
    Integer[] intList1 = {0,0,0,3};
    Integer[] intList2 = {1,2,3,3};
    Integer[] intList3 = {6,2,5,4};
    items.add(new ArrayList<Integer>(Arrays.asList(intList1)));
    items.add(new ArrayList<Integer>(Arrays.asList(intList2)));
    items.add(new ArrayList<Integer>(Arrays.asList(intList3)));

Or these int arrays can be added to the ArrayList in a loop, instead of individually.

D. Pereira · Accepted Answer · 2015-05-21 19:18:40Z

From what I understood, in each iteration of your loop, you will have an index and an integer. You want to insert the integer into the ArrayList at the specified index, if one exists. Or are you looping over a 2-dimensional array?

Consider using a HashMap where the key is the index (Integer), and the value is an ArrayList:

HashMap<Integer, ArrayList<Integer>> items = new HashMap<>();

// Given an index, i, and an integer, n 
// (e.g. obtained while looping over your data):

if (items.containsKey(i)){
    // Your HasMap contains an ArrayList at this index / key
    // Add the number to the end of the list

    items.get(i).add(n);

} else {
    // There is no ArrayList stored yet at this index / key
    // Create a new list containing only the number

    ArrayList<Integer> list = new ArrayList<>();
    list.add(n);

    items.put(i, list);
}

Thomas Shields · Accepted Answer · 2015-05-21 18:24:43Z

0

items.get(i).add(j);

where i and j are integers.

answered May 21, 2015 at 18:24

Thomas Shields

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2 Comments

Christopher Over a year ago

This works even if the index doesn't exists? (on the first addition)

Thomas Shields Over a year ago

No, you'd need to create it first by adding an empty ArrayList

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