Keep tests concrete

This popped up on a technical discussion site recently. The original question was how to write tests for code that invokes a method on particular values in a list. The problem was that the tests were messy, and the author was looking for a cleaner alternative. Here’s the example test, it asserts that the even-positioned elements in the parameters are passed to bar in the appropriate sequence.

public void testExecuteEven() {
  Mockery mockery = new Mockery();

  final Bar bar = mockery.mock(Bar.class);
  final Sequence sequence = new NamedSequence("sequence");

  final List<String> allParameters = new ArrayList<String>();
  final List<String> expectedParameters = new ArrayList<String>();

  for (int i = 0; i < 3; i++) {
    allParameters.add("param" + i);
    if (i % 2 == 0) {
      expectedParameters.add("param" + i);
    }
  }
  final Iterator<String> iter = expectedParameters.iterator();

  mockery.checking(new Expectations() {
   {
     while (iter.hasNext()) {
       one(bar).doIt(iter.next());
        inSequence(sequence);
      }
    }
  });

  Foo subject = new Foo();
  subject.setBar(bar);
  subject.executeEven(allParameters);
  mockery.assertIsSatisfied();
}

The intentions of the test are good, but its most striking feature is that there’s so much computation going on. This doesn’t need a new technique to make it more readable, it just needs to be simplified.

A unit test should be small and focussed enough that we don’t need any general behaviour. It just has to deal with one example, so we can make it as concrete as we like. With that in mind, we can collapse the test to this:

public void testCallsDoItOnEvenIndexedElementsInList() {
  final Mockery mockery = new Mockery();
  final Bar bar = mockery.mock(Bar.class);
  final Sequence evens = mockery.sequence("evens");

  final  List<String> params = 
    Arrays.asList("param0", "param1", "param2", "param3");

  mockery.checking(new Expectations() {{
    oneOf(bar).doIt(params.get(0)); inSequence(evens);
    oneOf(bar).doIt(params.get(2)); inSequence(evens);
  }});

  Foo subject = new Foo();
  subject.setBar(bar);
  subject.executeEven(params);
  mockery.assertIsSatisfied();
}

To me, this is more direct, a simpler statement of the example—if nothing else, there’s just less code to understand. I don’t need any loops because there aren’t enough values to justify them. The expectations are clearer because they show the indices of the elements I want from the list (an alternative would have been to put in the expected values directly). And if I pulled the common features, such as the mockery and the target object, into the test class, the test would be even shorter.

The short version of this post is: be wary of any general behaviour written into a unit test. The scope should be small enough that values can be coded directly. Be especially wary of anything with an if statement. If the data setup is more complicated, then consider using a Test Data Builder.