Java - Is ArrayList<Integer>[][] possible?

Question

ArrayList<Integer>[][] matrix = new ArrayList<Integer]>[sizeX][sizeY]();

or

ArrayList<Integer>[][] matrix = new ArrayList<Integer]>()[sizeX][sizeY];

don't work, I'm starting to think that it's not even possible to store ArrayLists in a matrix?

What do you want to achieve? The two things you posted are clearly not possible, as they don't compile. Could you tell us what you want?

user835259 · Accepted Answer · 2011-07-25 11:15:46Z

up vote 4 down vote accepted

If you still want to use and array:

    ArrayList<Integer>[][] matrix = new ArrayList[1][1];
    matrix[0][0]=new ArrayList<Integer>();
    //matrix[0][0].add(1);

answered Jul 25 '11 at 11:15

user835259
562

Jigar Joshi · Answer 2 · 2011-07-25 11:07:51Z

up vote 1 down vote

Try

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

Read more on List

answered Jul 25 '11 at 11:07

Jigar Joshi
78.6k983144

	What I'm trying to do is to create a multidimensional array (matrix) where I have an ArrayList on each position. – Sane Jul 25 '11 at 11:15
	ok. this is perfect for you then . – Jigar Joshi Jul 25 '11 at 11:18
	Hard to be sure, but it sounds like the OP actually wants a 3d matrix: a 2d matrix whose cells are arraylists. – CPerkins Jul 25 '11 at 12:14

bomslang · Answer 3 · 2011-07-25 11:09:15Z

up vote 1 down vote

Use this,

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

It means that you list will be consisting of List of Integers as its value.

answered Jul 25 '11 at 11:09

bomslang
15k53266

Michael Borgwardt · Answer 4 · 2011-07-25 11:13:11Z

up vote 0 down vote

Generics and arrays generally don't mix well, but this will work (gives a warning, which can be safely ignored):

ArrayList<Integer>[][] matrix = new ArrayList[sizeX][sizeY];

answered Jul 25 '11 at 11:13

Michael Borgwardt
147k16188392

Geziefer · Answer 5 · 2011-07-25 11:13:56Z

If you want to store a list in an array then you still have to separate the declaration and the inizialization:

ArrayList<Integer>[][] matrix = new ArrayList[10][10];

will specify a 2-dim-array of ArrayList objects.

matrix[0][0] = new ArrayList<Integer>();

will initialize one specific cell with a new ArrayList of Integers.

5 Answers