Recursive square matrix multiplication in java using index calculations: https://riyafa.wordpress.com/?p=958

MatrixMultiplicationRecursive.java

import java.util.Arrays;

public class MatrixMultiplicationRecursive {

    public static void main(String[] args) {
        int[][] A = new int[][]{{13, -3, -25, 20}, {-3, -16, -23, 18}, {20, -7, 12, -5}, {-22, 15, -4, 7}};
        int[][] B = new int[][]{{13, 10, 11, 12}, {13, 14, -23, 18}, {20, -7, 12, -11}, {-12, -13, -14, 7}};
        System.out.println(Arrays.deepToString(
                multiply(A, B, 0, 0, 0, 0, A.length)
        ));
    }

    public static int[][] multiply(int[][] A, int[][] B, int rowA, int colA,
                                   int rowB, int colB, int size) {
        int[][] C = new int[size][size];
        if (size == 1) {
            C[0][0] = A[rowA][colA] * B[rowB][colB];
        } else {
            int newSize = size / 2;
            // C11 = A11 * B11 + A12 * B21
            add(C, multiply(A, B, rowA, colA, rowB, colB, newSize),//A11*B11
                multiply(A, B, rowA, colA + newSize, rowB + newSize, colB, newSize), //A12*B21
                0, 0);//C11

            // C12 = A11 * B12 + A12 * B22
            add(C, multiply(A, B, rowA, colA, rowB, colB + newSize, newSize),//A11*B12
                multiply(A, B, rowA, colA + newSize, rowB + newSize, colB + newSize, newSize),//A12*B22
                0, newSize);//C12

            // C21 = A21 * B11 + A22 * B21
            add(C, multiply(A, B, rowA + newSize, colA, rowB, colB, newSize),//A21*B11
                multiply(A, B, rowA + newSize, colA + newSize, rowB + newSize, colB, newSize),//A22*B21
                newSize, 0);//C21

            // C22 = A21 * B12 + A22 * B22
            add(C, multiply(A, B, rowA + newSize, colA, rowB, colB + newSize, newSize),//A21*B12
                multiply(A, B, rowA + newSize, colA + newSize, rowB + newSize, colB + newSize, newSize), //A22*B22
                newSize, newSize);//C22
        }

        return C;
    }

    /**
     * @param C    Matrix of size n x n to add to
     * @param A    A matrix of size n/2 x n/2 to add from
     * @param B    A matrix of size n/2 x n/2 to add from
     * @param rowC The starting row of the C matrix
     * @param colC The starting column of the C matrix
     */
    private static void add(int[][] C, int[][] A, int[][] B, int rowC, int colC) {
        int n = A.length;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                C[i + rowC][j + colC] = A[i][j] + B[i][j];
            }
        }

    }
}