Suppose you've hired a company to deliver to you some software and they promised they unit test their code with 90% coverage, which seems to be a great deal.
So you decide to insert errors in these tests, so you expect to see a much lower coverage of passing tests, but, well, after inserting the errors, you discover that it still 90% coverage :-)
How useful are these tests?
for example, is this test right?
@Test
public void testAdd() {
int result = 0;
Claszz c = new Claszz();
int result2 = c.add(-1, 1);
assertEquals(result,0);
}