Result of the lesson demo (26.10.2016) Points: meaningful names, preconditions, small function, function preconditions, using domain vocabulary, code formatting.

MatrixUtils.java

import java.util.Arrays;

public class MatrixUtils {
    public static int[][] multiplyMatrix(
            int[][] left,
            int[][] right) {
        if (!checkMatrix(left) || !checkMatrix(right)) {
            throw new IllegalArgumentException();
        }
        if (getColumnCount(left) != getRowCount(right)) {
            throw new IllegalArgumentException();
        }
        int[][] result = new int[getRowCount(left)][getColumnCount(right)];
        for (int rowNumber = 0; rowNumber < getRowCount(left); rowNumber++) {
            for (int columnNumber = 0; columnNumber < getColumnCount(right); columnNumber++) {
                result[rowNumber][columnNumber] =
                        multiplyVector(getRow(left, rowNumber),
                                getColumn(right, columnNumber));
            }
        }
        return result;
    }

    private static int multiplyVector(int[] left, int[] right) {
        return 42;
    }

    private static int[] getColumn(int[][] matrix, int columnNumber) {
        int[] result = new int[getRowCount(matrix)];
        for (int rowNumber = 0; rowNumber < getRowCount(matrix); rowNumber++) {
            result[rowNumber] = matrix[rowNumber][columnNumber];
        }
        return result;
    }

    private static int[] getRow(int[][] matrix, int rowNumber) {
        return matrix[rowNumber];
    }

    private static int getRowCount(int[][] matrix) {
        return matrix.length;
    }

    private static int getColumnCount(int[][] matrix) {
        return matrix[0].length;
    }

    private static boolean checkMatrix(int[][] matrix) {
        return true;
    }

    public static boolean testMultiplyMatrix() {
        return Arrays.deepEquals(multiplyMatrix(
                new int[][]{{7, 4, 88},
                        {3, 554, 58}},
                new int[][]{{17, 4},
                        {3, 54},
                        {3, 54}}),
            new int[][]{{7, 4, 88},
                        {3, 554, 58}});
    }
}