NewInstance

Sometime it is very useful to create tests dynamically. For instance, I had to test that every URL returned by a method is valid and the content can be downloaded. In this case I don't know how many URL the method was returning, and I wanted to have a separate and independent test for every URL so that I can catch all the errors in a single run. If you create a single test with a loop, the first failure will stops the other tests to run. This approach worked fine: I caught 5 type of errors in over 2000 tests running on that method (one per url), some url was returning http status 404, some 500, 403, etc... everything documented in a test report, and promptly reported in the bug tracker.

This example, on the usual Adder, is about how to dynamically create tests:

 1public class TestAdd extends TestCase {
 2    private Adder adder;
 3    private long expected;
 4
 5    public TestAdd(int a, int b, long expected) {
 6        super("test" + a + "addedTo" + b + "equals" + expected);
 7        this.adder = new Adder(a, b);
 8        this.expected = expected;
 9    }
10
11    protected void runTest() throws Throwable {
12        assertEquals(expected, adder.sum());
13    }
14
15    public static Test suite() {
16        TestSuite suite = new TestSuite(TestAdd.class.getName());
17        Random r = new Random();
18        for(int i = 0; i < 10; i++) {
19            int a = r.nextInt(100);
20            int b = r.nextInt(100);
21            long expected = a + b;
22            suite.addTest(new TestAdd(a, b, expected));
23        }
24        return suite;
25    }
26}

I don't know if this can be so easily done with JUnit4 and annotations. I still prefer JUnit 3, I think it's much easier.