How many unit tests should I write per function/method?

Question

Do you write one test per function/method, with multiple checks in the test, or a test for each check?

Answer 1

In Java/Eclipse/JUnit I use two source directories (src and test) with the same tree. If I have a src/com/mycompany/whatever/TestMePlease with methods worth testing (e.g. deleteAll(List stuff) throws MyException) I create a test/com/mycompany/whatever/TestMePleaseTest with methods to test differente use case/scenarios:

@Test
public void deleteAllWithNullInput() { ... }

@Test(expect="MyException.class") // not sure about actual syntax here :-P
public void deleteAllWithEmptyInput() { ... }

@Test
public void deleteAllWithSingleLineInput() { ... }

@Test
public void deleteAllWithMultipleLinesInput() { ... }

Having different checks is simpler to handle for me.

Nonetheless, since every test should be consistent, if I want my initial data set to stay unaltered I sometimes have, for example, to create stuff and delete it in the same check to insure every other test find the data set pristine:

@Test
public void insertAndDelete() { 
    assertTrue(/*stuff does not exist yet*/);
    createStuff();
    assertTrue(/*stuff does exist now*/);
    deleteStuff();
    assertTrue(/*stuff does not exist anymore*/);
}

Don't know if there are smarter ways to do that, to tell you the truth...