Arrays fill with multidimensional array in Java

Question

Without using a loop, how can I fill a multidimensional array in Java? I tried:

double[][] arr = new double[20][4];
Arrays.fill(arr, 0);

The outcome is java.lang. ArrayStoreException: Double in Java

Answer 1

This is so that you don't accidentally assign 0.0 to a double[][] array of double[]s (which would be equivalent to double[] vector = 0.0). Java doesn't actually have any proper multidimensional arrays.

In Java, 0.0 is the default value for doubles, so when you retrieve the matrix from new, it will actually already be zeroed out. However, you could do the following if you wanted to fill it with, let's say, 1.0:

I don't think the API offers a way to deal with this issue without using a loop. However, using a for-each loop to complete the task is straightforward.

double[][] matrix = new double[20][4];

// Fill each row with 1.0
for (double[] row: matrix)
    Arrays.fill(row, 1.0);