Java equivalent to python array initialization

Question

What is the equivalent in Java to this in Python?

array_name = [0 for i in range(5)]

Specifically, I want to create a 2 dimensional Java ArrayList and initialize each value as 0. This is what I was thinking, in semi-psuedocode, following Python, but it obviously doesn't work:

ArrayList<ArrayList<Integer>> grid = new ArrayList<ArrayList<Integer>>() {{
    (add(Arrays.asList(0 for i : ROWS) for j : COLUMNS);
}};

Where ROWS and COLUMNS are constants defined elsewhere. In the end, I want a 2D ArrayList, with sizes given by the ROWS and COLUMNS constants, and each value initialized as 0. Is there a way to do this?

And ArrayLists don't allow primitive types as the generic type (like the primitive int). Then having the reference type doesn't have a default of 0 since it is an object. So any other questions concerning the initialization of an array to 0 that have to do with the primitive int don't answer mine.

to create an array of integers for example initializing, java does with zeros — Dev. Joel
– Dev. Joel, Commented Sep 18, 2016 at 22:18
@Dev.that's true for int[] but not for ArrayList<Integer>. — f1sh
– f1sh, Commented Sep 18, 2016 at 22:21
Possible duplicate of Any shortcut to initialize all array elements to zero? — Natecat
– Natecat, Commented Sep 18, 2016 at 22:21
Like @f1sh says, the possible duplicate is right for the primitive int and array, not my situation with the Integer object for an ArrayList. — Shivashriganesh Mahato
– Shivashriganesh Mahato, Commented Sep 19, 2016 at 0:08

Community · Accepted Answer · 2017-05-23 11:53:09Z

1

To initialize a ArrayList with zeros could make use of the method nCopies

    int ROWS =3;
    int COLUMNS=3;
    List<List<Integer>> twoDimArray = new ArrayList<List<Integer>>();//declaration
    for (int i = 0; i < ROWS ; i++) { // cant rows 
        twoDimArray.add(Collections.nCopies(COLUMNS, new Integer(0)));
        //initialized to zero the number of columns you want
    }

    /* Print  */

    for (List<Integer> list : twoDimArray) {
        System.out.println(list);
    }

In the example use the List <> to replace ArrayList as it is much better ... and for references to this review Type List vs type ArrayList in Java

edited May 23, 2017 at 11:53

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answered Sep 18, 2016 at 23:07

Dev. Joel

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5 Comments

Shivashriganesh Mahato Over a year ago

What exactly does the "cant" mean in the two variables declared at the top?

Dev. Joel Over a year ago

Is the number of rows and columns of your multidimensional array

Shivashriganesh Mahato Over a year ago

So pretty much cantRows is ROWS and cantColumns is COLUMNS in my code?

Dev. Joel Over a year ago

If it is, change my answer to that is more understandable

auntyellow Over a year ago

Integer.valueOf(0) may save heap than new Integer(0)

auntyellow · Accepted Answer · 2016-09-18 22:21:28Z

1

ArrayList<ArrayList<Integer>> grid = new ArrayList<>();
for (int i = 0; i < ROWS; i ++) {
    ArrayList<Integer> row = new ArrayList<>();
    for (int j = 0; j < COLUMNS; j ++) {
        row.add(Integer.valueOf(0));
    }
    grid.add(row);
}

answered Sep 18, 2016 at 22:21

auntyellow

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Comments

smac89 · Accepted Answer · 2019-11-12 21:49:11Z

In java 8, the equivalent would be:

List<List<Integer>> grid = Stream.generate(() -> Collections.nCopies(COLUMNS, 0))
                                 .limit(ROWS)
                                 .collect(Collectors.toList());

Note that you cannot modify the list returned by Collections.nCopies, so a better alternative might be:

List<List<Integer>> grid = Stream.generate(() -> new ArrayList<>(Collections.nCopies(COLUMNS, 0)))
                                 .limit(ROWS)
                                 .collect(Collectors.toList());

If you really need the type of the grid to be ArrayList<ArrayList>, you do:

ArrayList<ArrayList<Integer>> grid = Stream.generate(() -> new ArrayList<>(Collections.nCopies(COLUMNS, 0)))
                                           .limit(ROWS)
                                           .collect(Collectors.toCollection(ArrayList::new));

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