如何在Java中动态地正确初始化多维数组？

Question

Q. How to initialize arrays dynamically in Java?

I'm trying to store some metrics in arrays by using the following code.

 public static void main (String[] args) {
    Scanner in = new Scanner(System.in);
    int t = in.nextInt(); // outer metric size 
    int [] n = new int[t]; // inner square metric e.g. 3x3
    int [][][] a = new int[t][][]; // e.g. 2x3x3, 10x3x3

    //input block
    for (int h=0; h<t; h++){
        n[h] = in.nextInt(); //inner square metric dimensions
        for (int i=0;i<n[h];i++){
            for (int j=0;j<n[h];j++){
                a[h][i][j] = in.nextInt();    //metric values
            }
        }
    }

results in Null Pointer Exception which in turn is an array reference expected error. Changing the arrays to fixed size, doesn't cause this issue as expected.

  int [] n = new int[70];
  int [][][] a = new int[70][10][10];

Therefore, I would like to understand the right way to initialize dynamic arrays.

Answer 1

You have to allocate a new int[][] in the outer loop. Something like,

n[h] = in.nextInt(); //inner square metric dimensions
a[h] = new int[n[h]][n[h]]; //add this