Please create 3 different unit tests on the following methods in JAVA and explain how each unit test works. The tests should detect few bugs if found.

public Matrix multiplyWithMatrix(Matrix b, Matrix res) { if ((b == null) || (b == null)) { return null; } int n = b.getWidth(); if ((b.getHeight() != width) || (res.getHeight() != height) || (res.getWidth() != n)) { return null; } // Multiplication for (int i = 0; i < height; i++) { for (int j = 0; j < n; j++) { res.matrix[i][j] = 0; for (int k = 0; k < width; k++) { res.matrix[i][j] += matrix[i][k] * b.matrix[k][j]; } } } return res; }