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JUnit, Hamcrest, and jMock allow us to specify just what we want from the target code (there are equivalents in other languages) The more precise we are, the more the code can ex in other unrelated dimensions without breaking tests misleadingly Our experience is that the other bene t of keeping tests exible is that they re easier for us to understand because they are clearer about what they re testing about what is and is not important in the tested code
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A test might need to pass a value to trigger the behavior it s supposed to exercise in its target object The value could either be passed in as a parameter to a method on the object, or returned as a result from a query the object makes on one of its neighbors stubbed by the test If the test is structured in terms of how the value is represented by other parts of the system, then it has a dependency on those parts and will break when they change For example, imagine we have a system that uses a CustomerBase to store and nd information about our customers One of its features is to look up a Customer given an email address; it returns null if there s no customer with the given address
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public interface CustomerBase { // Returns null if no customer found Customer findCustomerWithEmailAddress(String emailAddress); [ ] }
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When we test the parts of the code that search for customers by email address, we stub CustomerBase as a collaborating object In some of those tests, no customer will be found so we return null:
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allowing(customerBase)findCustomerWithEmailAddress(theAddress); will(returnValue(null));
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There are two problems with this use of null in a test First, we have to remember what null means here, and when it s appropriate; the test is not self-explanatory The second concern is the cost of maintenance Some time later, we experience a NullPointerException in production and track the source of the null reference down to the CustomerBase We realize we ve broken one of our design rules: Never Pass Null between Objects Ashamed, we change the CustomerBase s search methods to return a Maybe type, which implements an iterable collection of at most one result
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public interface CustomerBase { Maybe<Customer> findCustomerWithEmailAddress(String emailAddress); } public abstract class Maybe<T> implements Iterable<T> { abstract boolean hasResult(); public static Maybe<T> just(T oneValue) { public static Maybe<T> nothing() { }
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We still, however, have the tests that stub CustomerBase to return null, to represent missing customers The compiler cannot warn us of the mismatch because null is a valid value of type Maybe<Customer> too, so the best we can do is to watch all these tests fail and change each one to the new design If, instead, we d given the tests their own representation of no customer found as a single well-named constant instead of the literal null, we could have avoided this drudgery We would have changed one line:
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public static final Customer NO_CUSTOMER_FOUND = null;
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public static final Maybe<Customer> NO_CUSTOMER_FOUND = Maybenothing();
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without changing the tests themselves Tests should be written in terms of the information passed between objects, not of how that information is represented Doing so will both make the tests more self-explanatory and shield them from changes in implementation controlled elsewhere in the system Signi cant values, like NO_CUSTOMER_FOUND, should be de ned in one place as a constant There s another example in 12 when we introduce UNUSED_CHAT For more complex structures, we can hide the details of the representation in test data builders ( 22)
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In a test, focus the assertions on just what s relevant to the scenario being tested Avoid asserting values that aren t driven by the test inputs, and avoid reasserting behavior that is covered in other tests We nd that these heuristics guide us towards writing tests where each method exercises a unique aspect of the target code s behavior This makes the tests more robust because they re not dependent on unrelated results, and there s less duplication Most test assertions are simple checks for equality; for example, we assert the number of rows in a table model in Extending the Table Model (page 180) Testing for equality doesn t scale well as the value being returned becomes more
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