java-tutorials/java-math/src/main/java/com/baeldung/matrices/HomemadeMatrix.java

package com.baeldung.matrices;

public class HomemadeMatrix {
    public static double[][] multiplyMatrices(double[][] firstMatrix, double[][] secondMatrix) {
        double[][] result = new double[firstMatrix.length][secondMatrix[0].length];

        for (int row = 0; row < result.length; row++) {
            for (int col = 0; col < result[row].length; col++) {
                result[row][col] = multiplyMatricesCell(firstMatrix, secondMatrix, row, col);
            }
        }

        return result;
    }

    private static double multiplyMatricesCell(double[][] firstMatrix, double[][] secondMatrix, int row, int col) {
        double cell = 0;
        for (int i = 0; i < secondMatrix.length; i++) {
            cell += firstMatrix[row][i] * secondMatrix[i][col];
        }
        return cell;
    }
}