java - Method deepHashCode of Arrays class on two dimensional array

Question

I have some problems understanding the behavior of Arrays.deepHashCode on int[][]. I assume the program is easy enough to read. A and B has equal hash as expected. D is also different from the rest. But for some reason C is equal to A and B. C is actually transpose of arrayA (and arrayB), but that may just be a coincidence and not the reason for same hash code returned.

Am I not supposed to use this method on int[][]? Any suggestions of a good way to make my own hashCode for int[][] that returns equal integer if and only if each element in each subarray are equal to each other (same position, same length...like deep cloned)

import java.util.Arrays;
public class Test{
    public static void main(String[] args){

            int[][] arrayA = new int[][]{
                {1,2,3},
                {4,5,6},
                {7,8,9}
            };

            int[][] arrayB = new int[][]{
                {1,2,3},
                {4,5,6},
                {7,8,9}
            };

            int[][] arrayC = new int[][]{
                {1,4,7},
                {2,5,8},
                {3,6,9}
            };

            int[][] arrayD = new int[][]{
                {1,5,9},
                {2,3,7},
                {4,6,8}
            };

            System.out.println("Deep hash codes:");
            System.out.println("Array A: " + Arrays.deepHashCode(arrayA));
            System.out.println("Array B: " + Arrays.deepHashCode(arrayB));
            System.out.println("Array C: " + Arrays.deepHashCode(arrayC));
            System.out.println("Array D: " + Arrays.deepHashCode(arrayD));
    }
}

Output:

Deep hash codes:

Array A: 30729379

Array B: 30729379

Array C: 30729379

Array D: 30760099

Answer 1

Any suggestions of a good way to make my own hashCode for int[][] that returns equal integer if and only if each element in each subarray are equal to each other (same position, same length...like deep cloned)

That is impossible.

There are only so many different integers. Your array of integers has a lot more combinations. So there will be collisions.

All a hashCode can do is guarantee the opposite: If two arrays have a different hashCode, they are different.