Java array with empty second brackets

Question

In Java you can do this

int[][] i = new int[10][];

Does this just create 10 empty arrays of int? Does it have other implications?

Answer 1

It creates a 10-entry array of int[]. Each of those 10 array references will initially be null. You'd then need to create them (and because Java doesn't have true multidimensional arrays, each of those 10 int[] entries can be of any length).

So for instance:

int i[][] = new int [10][];
i[0] = new int[42];
i[1] = new int[17];
// ...and so on

Answer 2

Executing your code creates an array of size 10, each element of which can hold a reference to a int[], but which are all initialized to null.

In order to use the int[]s, you would have to create new int[] for each of the element, something like this:

for (int n = 0; n < 10; n++)
    i[n] = new int[10]; // make them as large as you need

Answer 3

Yes, it does; however, each of those arrays are null. You have to then initialize each of those sub-arrays, by saying int[10][0] = new int[MY_SIZE], or something similar. You can have arrays with different lengths inside the main array; for example, this code would work:

int[][] i = new int[10][];
for(int ind = 0; ind<10;ind++){
    i[ind]=new int[ind];
}

It is just an array of arrays.

Answer 4

Here you create ten new int[0] arrays. You have to manually initialize it, it's useful when you don't need square matrix:

    int[][] array = new int[10][];
    for (int i = 0; i < array.length; i++) {
        array[i] = new int[i];
    }

If you need square matrix you can do:

    int[][] array = new int[10][10];

And it will be initialized with default values.

Answer 5

That is just the declaration, you need to initialize it. The 10 arrays would be null initially.