如何为基于TDD的方法编写否定测试？

Question

I have following code and wrote two tests for the positive and negative conditions of it. But based on TDD I should write a negative test for the whole code as well how should I write that?

"There must be at least two unit test cases for each requirement: one positive test and one negative test. If a requirement has sub-requirements, each sub-requirement must have at least two test cases as positive and negative."

Code

public class A {

    boolean myOuput(int a) {
        System.err.println(a);

        if(a == 5)

            return true;
        else
            return false;
    }

Tests

public class ATest {
    static A a;
    public ATest() {
    }

    @BeforeClass
    public static void setUpClass() {
        a = new A();
    }

    @AfterClass
    public static void tearDownClass() {
        System.err.println("tearDown class");
    }

    @Before
    public void setUp() {
        System.err.println("setUp");
    }

    @After
    public void tearDown() {
        System.err.println("tearDown");
    }


    @Test
    public void testOutputNotFive(){

        assertEquals(false,a.myOuput(0));
    }

    @Test
    public void testOutputForFive(){

        assertEquals(true,a.myOuput(5));
    }

}

Answer 1

I'm not sure where you are quoting from, but I'm doing TDD since a few years and would write these two tests:

shouldReturnTrueIfInputIsFive() {...}

shouldReturnFalseIfInputIsNotFive() {...}

This covers 100% of your code - which is all you want :)

EDIT : As a side note - if you do TDD, you want tests to be as descriptive as possible (they actually replace the documentation). Having a clear naming pattern like should - what - when is very helpful, because if something like testOutputNotFive gives no indication about the desired outcome...

Answer 2

Your question shows a very simplistic example. In testing we often find errors occur around boundary conditions. On either side of a boundary condition you tend to have an equivalence partition. ie a range of values that return the same result. For example, an insurance policy may have a restriction on a persons age. The person must be 18-65 years old to take out the policy. In this example 18 and 65 are boundaries. You should write a test for 18 and 65. You should also write a test for 17 ie 1 less that the lower boundary (all values lower than 17 are on the same equivalence partition that starts at 17). You would need to do the same for 66. Testing 19 and 64 would ensure that you have also covered the inner equivalence partition.