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CHAPTER 12 UNIT TESTING ALGORITHMS
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assertTrue( addresses.hasOneCandidateOrOneIsDefiniteMatch() );
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Running this gives us another green bar so we re very nearly home and dry. There s just one more test scenario to go. This is the test method for the CandidateCollection containing several addresses, all of which are exact candidates scenario: /** * Input: CandidateCollection containing several addresses, all of which are exact * candidates * Acceptance Criteria: false returned */ @Test public final void checkSeveralExactCandidates() throws Exception { List<AddressCandidate> testData = new ArrayList<AddressCandidate>(3); testData.add(new AddressCandidate(true)); testData.add(new AddressCandidate(true)); testData.add(new AddressCandidate(true)); CandidateCollection addresses = new CandidateCollection(testData); assertFalse( addresses.hasOneCandidateOrOneIsDefiniteMatch() );
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This one should return false, even though all the address candidates are exact matches. The business rule requires that exactly one of them be an exact match. However, the result of running the test is shown in Figure 12 8.
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Figure 12 8. checkSeveralExactCandidates() produces a test failure.
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CHAPTER 12 UNIT TESTING ALGORITHMS
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To examine why this test failed, let s revisit the method under test here it is from earlier: public boolean hasOneCandidateOrOneIsDefiniteMatch() { if (candidates.size()==1) { return true; } for (AddressCandidate candidate : candidates) { if (candidate.isExactMatch()) { return true; } } return false; } As you can see, there s a quite blatant bug in there: there s an assumption that any address candidate with an exact match should result in true being returned. However, the test caught this wrong assumption. The rewritten code is as follows: public boolean hasOneCandidateOrOneIsDefiniteMatch() { if (candidates.size()==1) { return true; } boolean foundExactMatch = false; for (AddressCandidate candidate : candidates) { if (candidate.isExactMatch()) { if (foundExactMatch) { return false; } foundExactMatch = true; } } return foundExactMatch; } The idea behind this version is that if the first exact match is found, that s good, but if another exact match is found, the code immediately returns with false. Re-running the tests produces a pass. But seeing as this code caused us a bit of a problem, let s focus on it and see if there are further test techniques we can use to zoom in and peek inside the (supposedly atomic) function.
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Never imagine yourself not to be otherwise than what it might appear to others that what you were or might have been was not otherwise than what you had been would have appeared to them to be otherwise, said the Duchess.7
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One way to keep an eye on the code s internals and check that they re working as expected is to pass in a spy object. This is a bit like a mock object, but it s a tracking implementation of an object used by the code under test, to record various diagnostics, such as whether a particular method was called, how many times it was called, the order that the methods were called in, and the mid-algorithm, or transition state values that were passed around.
Tip Use spy objects to test for correct state transitions within functions.
Lewis Carroll, Alice s Adventures in Wonderland (1865), 9 The Mock Turtle s Story .
CHAPTER 12 UNIT TESTING ALGORITHMS
Looking at the fixed version of hasOneCandidateOrOneIsDefiniteMatch(), and the checkSeveralExactCandidates() test scenario which is passing in a collection of three exact matches we expect the overall size of the collection to be checked once (to see if it has exactly one match), and we expect candidate.isExactMatch() to be called twice. (That should be the case whether three exact matches are passed in or 300, as the code will exit as soon as a second exact match is found.) So we ll create two spy objects one for the java.util.ArrayList collection (the backing store behind CandidateCollection), and the other for AddressCandidate. In other words, see Figure 12 9.
Figure 12 9. Our new spy classes pretending to be the real ones while taking notes We ll create both as static inner classes inside the test class. Let s start with SpyList.
Spy Object 001: SpyList
Referring back to Figure 12 9, SpyList extends java.util.ArrayList with additional spy functionality. Here s our new SpyList inner class: static class SpyList<E> extends ArrayList<E> { public int size() { sizeCalled++; return super.size();
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