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 add(Matrix b, Matrix res) { if ((res == null) || (b == null) || (b.getHeight() != height) || (b.getWidth() != width) || (res.getHeight() != height) || (res.getWidth() != width)) { return res; } // Addition for (int i = 0; i < height; i++) { for (int j = 0; j < width; j++) { res.matrix[i][j] = matrix[i][j] + b.matrix[i][j]; } } return res; }