How do I allocate an array of arrays in Java?

Question

I want to dynamically allocate memory for an array of arrays, knowing that the number of rows is going to be n, but I don't want to allocate more memory for each row than needed, which is going to be i = 1:n, number of elements = i for each row, and I know this in advance.

    int [] a = new int[n];
    for (int i=0; i<n; i++)

I have just started to learn Java, and I'm new to this. As far as I know, the first line will allocate memory for n elements (number of rows), and what I want to do is create a new array of i elements at each iteration.

You should prefer using ArrayList if you need dynamism in size, although arrays perform better than ArrayList — Pushpesh Kumar Rajwanshi, Oct 12 '18 at 06:51
Possible duplicate of [How to make an array of arrays in Java](https://stackoverflow.com/questions/4781100/how-to-make-an-array-of-arrays-in-java) — teppic, Oct 12 '18 at 07:24

score 5 · Accepted Answer · answered Oct 12 '18 at 06:52

You can declare a two-dimensional array and allocate the first dimensional

  int [][] a = new int[n][];

And then, inside a loop, you can allocate the second one

 for (int i=0; i<n; i++)
   a[i] = new int[necessary_length];

But if you know the size in advance you obviously can declare it in the beginning

  int [][] a = new int[n][n];

How do I allocate an array of arrays in Java?

1 Answers1