Java Extreme Programming Cookbook

Unit Test Granularity

Problem

You want to know how fine-grained your unit tests should be.

Solution

Each unit test should check one specific piece of functionality. Do not combine multiple, unrelated tests into a single testXXX( ) method.

Discussion

Each test method can have as many assertXXX( ) calls as you like, but they can lead to problems:

public void testGame(  ) throws BadGameException {
    Game game = new Game(  );
    Ship fighter = game.createFighter("001");
    assertEquals("Fighter did not have the correct identifier",
        "001", fighter.getId(  ));
    Ship fighter2 = game.createFighter("001");
    assertSame("createFighter with same id should return same object",
        fighter, fighter2);
    assertFalse("A new game should not be started yet", game.isPlaying(  ));
}

This is a bad design because each assertXXX( ) method is testing an unrelated piece of functionality. If the first assertEquals( ) fails, the remainder of the test is not executed. When this happens, you won’t know if the other tests are functional.

Example 4-2 shows a refactored test case that tests various aspects of our game independently. We will see how to remove the duplicated code in Recipe 4.6 when we talk about setUp( ) and tearDown( ).

Example 4-2. Refactored tests

public void testCreateFighter( ) throws BadGameException { Game game = new Game( ); Ship fighter = game.createFighter("001"); assertEquals("Fighter did not have the correct identifier", "001", fighter.getId( )); game.shutdown( ); } public void testSameFighters( ) throws BadGameException ...