An example of power corrupting

Brett is currently with ThoughtWorks and he has numerous years of experience across a number of domains and disciplines including Training (from 1985), solution-based consulting (from 1992) and coaching (from 2000). He likes to both develop and train/coach, changing emphasis between the two every 4 – 6 years.

The positive effect modern mocking tools can have on our ability to work with legacy code and the possible negative implications of using those tools.

In Working Effectively with Legacy Code Michael Feathers defines legacy code as code without automated tests. Sometime in 2010 I was introduced to JMockIt and shown how it allows us to write automated tests that seemingly violate the semantics of Java. For example, it is possible to replace a static method during the execution of a test. If I were to use the "old" style suggested by Michael Feathers, I'd do something like introduce an instance delegator to make it possible to write the test I wanted to write. Now, with a modern mocking tool, I could just skip that step. My initial reaction was amazement as I saw this as a way to open up existing code that is hard to test and get something written faster and with less mucking about in existing code.

Fast forward to late 2011 where I was teaching a class in Berlin and asked to give a presentation at a user's group (you can see that talk here). In that video you can see me fuddle about trying to get some legacy code under test using JMockIt and in the end I manage to do so without actually changing the underlying code. What you don't see in the video is that next day in class we take the resulting code and apply more traditional legacy refactoring techniques to it and then rewrite the JMockIt-based test. The results are staggering and I believe they speak for themselves. What follows is that story recreated in full, which much of the fuddling about, I hope, removed.

The Usual Suspects To get started, here is some code for your consideration: public static BigDecimal convertFromTo(String fromCurrency, String toCurrency) { Map<String, String> symbolToName = currencySymbols(); if (!symbolToName.containsKey(fromCurrency)) throw new IllegalArgumentException(String.format( "Invalid from currency: %s", fromCurrency)); if (!symbolToName.containsKey(toCurrency)) throw new IllegalArgumentException(String.format( "Invalid to currency: %s", toCurrency)); String url = String.format("http://www.gocurrency.com/v2/dorate.php?inV=1&from=%s&to=%s&Calculate=Convert", toCurrency, fromCurrency); try { HttpClient httpclient = new DefaultHttpClient(); HttpGet httpget = new HttpGet(url); HttpResponse response = httpclient.execute(httpget); HttpEntity entity = response.getEntity(); StringBuffer result = new StringBuffer(); if (entity != null) { InputStream instream = entity.getContent(); InputStreamReader irs = new InputStreamReader(instream); BufferedReader br = new BufferedReader(irs); String l; while ((l = br.readLine()) != null) { result.append(l); } } String theWholeThing = result.toString(); int start = theWholeThing.lastIndexOf("<div id=\"converter_results\"><ul><li>"); String substring = result.substring(start); int startOfInterestingStuff = substring.indexOf("<b>") + 3; int endOfIntererestingStuff = substring.indexOf("</b>", startOfInterestingStuff); String interestingStuff = substring.substring( startOfInterestingStuff, endOfIntererestingStuff); String[] parts = interestingStuff.split("="); String value = parts[1].trim().split(" ")[0]; BigDecimal bottom = new BigDecimal(value); return bottom; } catch (Exception e) { throw new RuntimeException(e); } } Ostensibly this code screens scrapes currency conversion information in a rather obtuse way, however the real purpose of this code is to discuss poor coding choices and their effect on our ability to understand, test, and maintain this code. Challenges to Testing Before delving into the method itself, first observe that this is a static method. In Java, C#, C++ (and many other languages), static methods are always bound at compile (or by link) time. What this means is that code calling static methods is directly coupled to those methods. The selection of the method and the calling mechanism are selected too early to easily allow tests to setup an environment in which they can divert to something the test controls. It is possible to call a different static method at runtime using what Michael Feathers calls a "link seam." In the case of Java, you could make sure a different version of the class with the static method is available in the classpath earlier. There is more on this below. Validation The first thing this method does is perform some basic validation. It confirms that the symbols provided as method arguments actually exist. To do this, it calls another method in the same class. Map<String, String> symbolToName = currencySymbols(); if (!symbolToName.containsKey(fromCurrency)) throw new IllegalArgumentException(String.format( "Invalid from currency: %s", fromCurrency)); if (!symbolToName.containsKey(toCurrency)) throw new IllegalArgumentException(String.format( "Invalid to currency: %s", toCurrency)); Required The method to get the known symbols is static and in the same class. This makes it impossible within the language to simulate it or otherwise not perform validation. While it may seem a good idea to always perform validation, it does increase the testing burden, which makes tests somewhat more fragile as they depend on a larger number of things that could change over time. That is only scratching the surface. It might seem that always testing validation when testing other things makes the checks on validation more thorough, in fact such testing typically represents duplicated rather than deeper testing; it adds bulk without increasing value. All I want to do is verify that parsing is correct. As written, validation and parsing must be done together, in a particular order. That order seems logical; to have input I need valid currencies. However, while that may seem like a necessary constraint, nothing about parsing depends on valid currencies, nor should it. It just cares that the text to parse follows a form, not that part of that form happens to additionally include valid currency symbols. So the validation's business is bleeding unnecessarily into the next step. I'd say this is an example of unnecessary temporal coupling. While parsing does follow validation in time, it should not therefore follow that validation must be required to allow us to verify parsing. A given test knows what it is testing. It knows, for example, if the input it wants to be parsed is valid or not. So forcing validation before parsing is incidental to how the code was written. It is not essential for parsing. This last point is worth emphasizing as it is a common misconception. In my experience, most people would suggest that validation before parsing is essential. In a sense it is; to get some valid input, I need valid currency symbols in the real system. However, for the purpose of testing, I do not need the real system, so the requirement of passing validation is really incidental. If we check that validation works and we check that parsing works, then is there any likelihood that what we've written won't work? I think not and so I don't see the need for a fully-integrated functional test. Someone might disagree with me. On a real project I'd probably write one because the argument would take more time than just writing an automated check. However, breaking things down into smaller and smaller pieces is both an essential skill and it takes years to learn. However, even if you disagree with me on that point, I hope we can agree on is that if I could verify parsing independently from validation, that would make checking things easier. In An Introduction to General Systems Thinking , Weinberg describes the The Square Law of Computation , which you might be more familiar with as "divide an conquer." This code clearly does not follow any such rules and the unnecessary, incidental coupling is going to make things comparatively difficult. Calls currencySymbols() The call of the static method as mentioned is an issue, but more of an issue is that the method in question makes a call out of the system using the HttpClient, so calling it requires an internet connection. This call is necessary, or at least used by, validation. Is the existence of this static method essentially related to other parts of this method or incidentally related? Don't confuse "as written" with "necessary." Traditional Options To address these issues, we could do a number of things: Extract validation to its own class by usingSprout Class, or Extract Class

Make all methods in this class non-static and non private, and use a testing subclass to make testing easier

Use an instance delegator - where the static methods are left but internally they call an instance method on an object These solutions address an underlying language design issue (non-override-able static stuff is how they are designed but not fundamentally necessary, consider Smalltalk, JavaScript, Self, ...). In any case, which one to pick depends on many factors including the starting point, how much existing code currently depends on the class, etc. HttpClient Next, the code reads a web page using the HttpClient: HttpClient httpclient = new DefaultHttpClient(); HttpGet httpget = new HttpGet(url); HttpResponse response = httpclient.execute(httpget); HttpEntity entity = response.getEntity(); Directly Uses This code directly uses the HttpClient, and uses new to boot. Inheritance is the highest form of coupling in Java followed closely by calling new . As this code is written, there is no language-defined way to avoid the use of this class. You can use a link seam by testing with a different jar file (or classpath). In a pinch I'll consider doing that, but not if I have access to the code and can change it, or, as we'll see, access to what I'd call fourth generation mocking tool like JMockIt (Java-Open Source), powermock (Java-Open Source) or Isolator.Net (commercial, .Net) Violates Dependency Inversion In this example, the business domain is currency conversion but the business logic directly uses the HttpClient. This is a violation of theDependency Inversion Principle. In this case, if someone wants to get a currency conversion, as the code is written, trying to do so introduces direct, compile-time coupling to a class that needs a connection to the Internet. High-level stuff depends on low-level details. This code will not age as well as an alternative that tends to invert this kind of dependency. Fixing This Problem The options to address this are the same as with validation: introduce an instance method, sprout a class, etc. There's a deeper problem, however. Not only does this code directly depend on a connection to the internet, as we are about to see, it returns HTML, which must be parsed. We care about conversion rates, not parsing HTML, but to get to what we want we have to go through a number of technology layers and after all of that we have to deal with HTML. File I/O The HttpClient makes an InputStream available, which is then read to completion: StringBuffer result = new StringBuffer(); if (entity != null) { InputStream instream = entity.getContent(); InputStreamReader irs = new InputStreamReader(instream); BufferedReader br = new BufferedReader(irs); String l; while ((l = br.readLine()) != null) { result.append(l); } } String theWholeThing = result.toString(); Embedded As with the previous sections, this code is directly embedded in the method, making it harder to ignore it. Complexity & Duplication This code is not terribly complex but to know what it is doing, you have to read it. One way to improve communication is to reduce the need for it and the same can be said for code. As this code is written, you have to read it to understand it. If this were in its own method or class, with a good name, it might be easier to pass over. Since we tend to read code more than write it, anything we can do to reduce the need to read code is time well invested in the life of a project. Required to get through the method This code is embedded and as written, it must be executed every time to exercise the code. This is another example of temporal coupling. As hinted to in the previous section, if this were organized in a way that made it possible to better understand it, then it might also be easier to get rid of it when the thing we are trying to check is not directly related to reading the contents of a stream. Parsing Now that the stream has been converted into a string, it's time to parse the results: int start = theWholeThing.lastIndexOf("<div id=\"converter_results\"><ul><li>"); String substring = result.substring(start); int startOfInterestingStuff = substring.indexOf("<b>") + 3; int endOfIntererestingStuff = substring.indexOf("</b>", startOfInterestingStuff); String interestingStuff = substring.substring( startOfInterestingStuff, endOfIntererestingStuff); String[] parts = interestingStuff.split("="); String value = parts[1].trim().split(" ")[0]; Third verse same as the first... At this point you might notice that I'm sounding like a skipping record. This code has all of the issues mentioned above: it must be executed as written, it violates dependency inversion, it must be read to be understood. SRP Violation This method is a classic demonstration of violating the Single Responsibility Principle. It does many different things, each of which have different reasons to change at different times. In fact, in the original form of this method, I was hitting one web site, which became unavailable, so I had to change it to another site to get the information I wanted. This broke things in several places, speaking towards the issues with violation of SRP and DIP. Not only did I need to hit a different location (HttpStuff), I was getting back different HTML (parsing) and I had to for a different URL (Http stuff again).

Diving In Modern mocking tools make most of the things I mention above irrelevant; at least on the surface. Rather than try to fix these things in the production code first, let's dive in by trying to get this method executing via an automated unit test. Getting setup - exercise the code One place to start is to simply try to execute the code in question with either null arguments or "reasonable" values. Since the domain is currency conversion, and the method takes in two currencies, that seems like a reasonable starting place. Here is the beginning of an automated unit test to simply exercise the code. My goal is to get through the method. One note, while this code will run with a live internet connection, I've turned off my connection while writing this test to make sure I do not require a live internet connection for my test to work: @Test public void returnExpectedConversion_v1() { CurrencyConversion.convertFromTo("USD", "EUR"); } This works if the network is enabled, but if not, the code generates a java.net.UnknownHostException . However, the actual place where it happens is in the method CurrencyConversion.currencySymbols , and not in the method we care about. Using older tools, this is a bit of work, not so with our tool of choice for this article: JMockIt Getting past validation Here is a second version of the test that gets past the first exception: @Test public void returnExpectedConversion_v2() { new NonStrictExpectations(CurrencyConversion.class) { { CurrencyConversion.currencySymbols(); result = mapFrom("USD", "EUR"); } }; CurrencyConversion.convertFromTo("USD", "EUR"); } private Map<String, String> mapFrom(String... keyValuePairs) { Map<String, String> result = new ConcurrentHashMap<String, String>(); for (int i = 0; i < keyValuePairs.length; ++i) result.put(keyValuePairs[i], keyValuePairs[i]); return result; } Running this, the same exception is thrown, UnknownHostException , but the exception is now in the method under test rather than a called method. That's an improvement. This allows the code to get past validation, but how? new NonStrictExpectations(CurrencyConversion.class) { { CurrencyConversion.currencySymbols(); result = mapFrom("USD", "EUR"); } }; Notice the creation of an anonymous inner class using NonStrictExpectations? This form tells JMockIt to do something with the CurrencyConversion class (replace a static method in this case). The code in the inner set of {} is a standard Java instance initializer. Code executed in that instance initializer tells JMockIt to replace the method executed, currencySymbols. This is an example of a partial mocking of the class; we replace one of the methods in the class such that any time it is called, it returns whatever is assigned to the inherited field "result". This involve some black magic. JMockIt is doing some Java bytecode magic and this code uses the JMockIt DSL to make this happen. Making this work involves adding the JMockIt jar file into the classpath. If you make sure it is listed before JUnit's jar file, that's enough to make this happen. JMockIt uses a JavaAgent registered in its MANIFEST.MF file to make this all automatic. Dealing with the client Now the code is failing on the part of the code that tries to read a webpage: HttpClient httpclient = new DefaultHttpClient(); HttpGet httpget = new HttpGet(url); HttpResponse response = httpclient.execute(httpget); HttpEntity entity = response.getEntity(); This code is a bit more complex since it is embedded in the method, but we can get past this problem. However, to do so will be a bit more work: The first two lines call new . We need to make those uses of the new operator return a class under our control.

. We need to make those uses of the new operator return a class under our control. The next line calls the execute method on an HttpClient , so we'll need to get that under control.

method on an , so we'll need to get that under control. The last line calls the getEntity method on an HttpResponse , which was the return value from the previous line, so this is much more involved overall. Here is one way to take care of these three issues in one fell swoop: new NonStrictExpectations() { DefaultHttpClient httpclient; HttpResponse response; HttpEntity entity; { httpclient.execute((HttpUriRequest) any); result = response; response.getEntity(); result = entity; entity.getContent(); result = bais; } }; In this use of NonStrictExpectations, there is no parameter passed in on the first line, meaning we are working with instances in some way rather than static stuff. This anonymous inner class has three fields: DefaultHttpClient, HttpResponse, HttpEntity. Those classes are wholly replaced in the Java classloader for this test only. This means, for example, calling new DefaultHttpClient will return an instance of the JMockIt-created class rather than the version found in the HttpClient jar. This is an example of what I call a dynamic link seam. It's a link seam like Michael Feathers discusses in his book, which is normally accomplished with build script/file magic. However, unlike using build scripts, this is using a library, which makes it available to Java code "within" the language, rather than outside of the language. This code replaces those three classes but with what? The HttpClient.execute method will always return response, which is an instance of the JMockIt-created HttpResponse subclass.

method will always return response, which is an instance of the JMockIt-created HttpResponse subclass. The HttpResponse.getEntity method will always return entity, which is an instance of the JMockIt-created HttpEntity subclass.

method will always return entity, which is an instance of the JMockIt-created HttpEntity subclass. The HttpEntity.getContent method will always return "bais", which we will see in the next section. Make no mistake, this is powerful stuff. In fact, I have a personal rule of thumb: If I think something is cool, then it may not be appropriate for real development. JMockIt makes my "cool spider senses" tingle on overdrive. Handling File I/O The underlying code needs the contents of a stream. To create this, the test uses some standard Java magic to make an in-memory stream: final ByteArrayInputStream bais = new ByteArrayInputStream( "<div id=\"converter_results\"><ul><li><b>1 USD = 0.98 EUR</b>" .getBytes()); This creates an in-memory ByteArrayInputStream from a string. How did I know what to put in the string? I had to reverse-engineer the underlying code. Even so, this makes the code execute. Putting it together Here is the test as a single method rather than broken up as it has been so far: @Test public void returnExpectedConversion_v3() throws Exception { final ByteArrayInputStream bais = new ByteArrayInputStream( "<div id=\"converter_results\"><ul><li><b>1 USD = 0.98 EUR</b>" .getBytes()); new NonStrictExpectations() { DefaultHttpClient httpclient; HttpResponse response; HttpEntity entity; { httpclient.execute((HttpUriRequest) any); result = response; response.getEntity(); result = entity; entity.getContent(); result = bais; } }; new NonStrictExpectations(CurrencyConversion.class) { { CurrencyConversion.currencySymbols(); result = mapFrom("USD", "EUR"); } }; BigDecimal result = CurrencyConversion.convertFromTo("USD", "EUR"); assertThat(result.subtract(new BigDecimal(2)), is(lessThanOrEqualTo(new BigDecimal(0.001)))); } On the one hand, this is pretty impressive. We've managed to exercise the code end-to-end without touching it. If we want to write something that uses this class, we now have a way to do it. Often, before trying to touch some code, we need to get it under test so we know if we broke anything after making changes. This is calledCharacterization Testing. Poorly written code like this typically makes doing so very hard. While it is still hard, at least we are able to do so without touching the production code. Having characterization tests around code makes refactoring safer. So we're done, right? Wrong.

Going Old School We've addressed the symptom, not the cause Notice that JMockIt made it possible to accomplish near black magic, but can we do better? If we try, will the time invested be worth the effort? In this section we start with the same method and make a few so-called legacy refactorings to the code to make it testable using more traditional tools, hand-rolled test doubles in this case. Then we'll compare, make an observation and then look at what would become possible. Introduce instance delegator A typical problem with static methods is they cannot be overridden. To fix this, we might simply make the class use all instance methods instead. However, let's assume for this example that we need to maintain backwards compatibility so we need to keep the static methods in place (in my experience this is not contrived). While I might try to do this test-first, in fact, I have an existing JMockIt test, so instead I'm going to simply make the necessary changes. I'll do this by copying CurrencyConversion into a new package with the name v2 added to it (the source for this blog is generated from source code, so I need to keep the original version around). To introduce an instance delegator: Introduce a static instance of the class.

Copy the static methods into instance methods (you'll need to create new method names as static methods and instance methods cannot have the same name and signature).

Change the static methods to call the instance methods on the internal static instance. Here's one way to do that (the lazy initialization of the internal instance is deliberate and if you're concerned about threading issues, we could use double-checked locking or just make the method synchronized): private static CurrencyConversion instance; private static CurrencyConversion getInstance() { if (instance == null) { instance = new CurrencyConversion(); } return instance; } public static BigDecimal convertFromTo(String fromCurrency, String toCurrency) { return getInstance().convert(fromCurrency, toCurrency); } public static Map<String, String> currencySymbols() { return getInstance().getAllCurrencySymbols(); } Extract a few methods There are several opportunities to extract some methods, so here's a version of v2/CurrencyConversion after a few method extractions: public BigDecimal convert(String fromCurrency, String toCurrency) { validateCurrencies(fromCurrency, toCurrency); try { String result = getPage(fromCurrency, toCurrency); String value = extractToValue(result); return new BigDecimal(value); } catch (Exception e) { throw new RuntimeException(e); } } protected void validateCurrencies(String fromCurrency, String toCurrency) { Map<String, String> symbolToName = currencySymbols(); if (!symbolToName.containsKey(fromCurrency)) throw new IllegalArgumentException(String.format( "Invalid from currency: %s", fromCurrency)); if (!symbolToName.containsKey(toCurrency)) throw new IllegalArgumentException(String.format( "Invalid to currency: %s", toCurrency)); } protected String extractToValue(String result) { String theWholeThing = result; int start = theWholeThing.lastIndexOf("<div id=\"converter_results\"><ul><li>"); String substring = result.substring(start); int startOfInterestingStuff = substring.indexOf("<b>") + 3; int endOfIntererestingStuff = substring.indexOf("</b>", startOfInterestingStuff); String interestingStuff = substring.substring( startOfInterestingStuff, endOfIntererestingStuff); String[] parts = interestingStuff.split("="); return parts[1].trim().split(" ")[0]; } protected String getPage(String fromCurrency, String toCurrency) throws URISyntaxException, IOException, HttpException { String url = String.format("http://www.gocurrency.com/v2/dorate.php?inV=1&from=%s&to=%s&Calculate=Convert", toCurrency, fromCurrency); HttpClient httpclient = new DefaultHttpClient(); HttpGet httpget = new HttpGet(url); HttpResponse response = httpclient.execute(httpget); HttpEntity entity = response.getEntity(); StringBuffer result = new StringBuffer(); if (entity != null) { InputStream instream = entity.getContent(); InputStreamReader irs = new InputStreamReader(instream); BufferedReader br = new BufferedReader(irs); String l; while ((l = br.readLine()) != null) { result.append(l); } } return result.toString(); } Here is a version of the last test targeting this class (the one in the v2 package). This test passes: @Test public void returnExpectedConversion_v4() throws Exception { final ByteArrayInputStream bais = new ByteArrayInputStream( "<div id=\"converter_results\"><ul><li><b>1 USD = 0.98 EUR</b>" .getBytes()); new NonStrictExpectations() { DefaultHttpClient httpclient; HttpResponse response; HttpEntity entity; { httpclient.execute((HttpUriRequest) any); result = response; response.getEntity(); result = entity; entity.getContent(); result = bais; } }; new NonStrictExpectations(CurrencyConversion.class) { { CurrencyConversion.currencySymbols(); result = mapFrom("USD", "EUR"); } }; BigDecimal result = CurrencyConversion.convertFromTo("USD", "EUR"); assertThat(result.subtract(new BigDecimal(2)), is(lessThanOrEqualTo(new BigDecimal(0.001)))); } Testing subclass Now I'll rewrite this test using hand-written test doubles instead JMockIt. Rather than show all of the intermediate steps, I'll just show a final result: class CurrencyConversion2_testingSubclass extends CurrencyConversion { @Override public void validateCurrencies(String fromCurrency, String toCurrency) { } @Override public Map<String, String> getAllCurrencySymbols() { return mapFrom("USD", "EUR"); } @Override public String getPage(String fromCurrency, String toCurrency) { return "<div id=\"converter_results\"><ul><li><b>1 USD = 0.98 EUR</b>"; } } @Test public void returnExpectedConversion_v5() throws Exception { CurrencyConversion original = CurrencyConversion.reset(new CurrencyConversion2_testingSubclass()); BigDecimal result = CurrencyConversion.convertFromTo("USD", "EUR"); assertThat(result.subtract(new BigDecimal(2)), is(lessThanOrEqualTo(new BigDecimal(0.001)))); CurrencyConversion.reset(original); } This test produces the same result as the JMockIt test doing much of the work by hand. Note, to make this possible, we need to allow for the new-created Singleton class to be set and reset: public static CurrencyConversion reset(CurrencyConversion other) { CurrencyConversion original = instance; instance = other; return original; }

Observations Actual amount of time to do this This technique of introducing an instance delegator, with an override-able static singleton and several extracted methods might seem like quite a bit. In practice this kind of change is quick. How quick? For this example, it took me 3 minutes using IntelliJ. Eclipse would have taken the same amount of time. In vi, maybe 1 minute (ok, maybe not, but I do use a vi plugin in Eclipse, IntelliJ and even Visual Studio). In any case, once you've practiced this, it's quick. We could have avoided much of this if the class had not used all static methods to begin with, but that's a common problem so knowing how to handle it is a good general technique to know. Yes, but... A common concern raised is what about making all of those extracted methods protected? It does not bother me because I believe test-ability is more important than design. That's actually a false dichotomy but it sounds controversial so I like to say it anyway. In fact, many of these "protected methods" are complex enough to warrant individual classes. Next, introduce dependency inversion and suddenly I'm wiring in dependent objects that are under my control, with override-able methods and checking individual parts of this overall flow become a snap. Divide and conquer. First test versus second test Here are the two tests again, for comparison: Using JMockIt @Test public void returnExpectedConversion_v4() throws Exception { final ByteArrayInputStream bais = new ByteArrayInputStream( "<div id=\"converter_results\"><ul><li><b>1 USD = 0.98 EUR</b>" .getBytes()); new NonStrictExpectations() { DefaultHttpClient httpclient; HttpResponse response; HttpEntity entity; { httpclient.execute((HttpUriRequest) any); result = response; response.getEntity(); result = entity; entity.getContent(); result = bais; } }; new NonStrictExpectations(CurrencyConversion.class) { { CurrencyConversion.currencySymbols(); result = mapFrom("USD", "EUR"); } }; BigDecimal result = CurrencyConversion.convertFromTo("USD", "EUR"); assertThat(result.subtract(new BigDecimal(2)), is(lessThanOrEqualTo(new BigDecimal(0.001)))); } Using Hand-rolled mocks and Refactoring class CurrencyConversion2_testingSubclass extends CurrencyConversion { @Override public void validateCurrencies(String fromCurrency, String toCurrency) { } @Override public Map<String, String> getAllCurrencySymbols() { return mapFrom("USD", "EUR"); } @Override public String getPage(String fromCurrency, String toCurrency) { return "<div id=\"converter_results\"><ul><li><b>1 USD = 0.98 EUR</b>"; } } @Test public void returnExpectedConversion_v5() throws Exception { CurrencyConversion original = CurrencyConversion.reset(new CurrencyConversion2_testingSubclass()); BigDecimal result = CurrencyConversion.convertFromTo("USD", "EUR"); assertThat(result.subtract(new BigDecimal(2)), is(lessThanOrEqualTo(new BigDecimal(0.001)))); CurrencyConversion.reset(original); } If you have to support test code, which version do you prefer? Wait, don't answer yet. First test still passes I want to reiterate that even though I made several changes to the production code, the JMockIt test still passes. That's pretty interesting to me. In fact, if you think in terms of how JMockIt manipulates the classloader, it makes sense. Even so, it's still pretty cool. What if we try with JMockIt again Why stop here? What happens if we take the time to rewrite the JMockIt test taking into consideration the changes made to the production code? @Test public void returnExpectedConversion_final() throws Exception { new NonStrictExpectations(CurrencyConversion.class) { CurrencyConversion c; { c.validateCurrencies(anyString, anyString); c.getPage(anyString, anyString); result = "<div id=\"converter_results\"><ul><li><b>1 USD = 0.98 EUR</b>"; } }; BigDecimal result = CurrencyConversion.convertFromTo("USD", "EUR"); assertThat(result.subtract(new BigDecimal(2)), is(lessThanOrEqualTo(new BigDecimal(0.001)))); } Now which one do you want to maintain? Notice, a little bit of refactoring makes the final JMockIt test much better. In fact, the amount of refactoring required to write this version of the test is smaller than the amount I did refactor. Simply extracting methods would have been enough to greatly improve the JMockIt test. Notice, however, that JMockIt did not force this. Not using a mocking tool or using a tool like Mockito would have forced me to do some amount of refactoring to get this class under test.

Closing the Feedback Loop What happens when people workopen loop? That is, you write some code and then tools like JMockIt make it possible to go back in and write tests after the fact without having to suffer for poor decisions? I ask because this example demonstrates such a situation. The original code is a mess, JMockIt allowed me to write a characterization test to exercise the code. That is great. It did not, however, force me to address the original problem. In a sense, it also allowed me to avoid the pain associated with having to fix a mess. If we take away the powerful tool, then we have to clean up the mess to get a test written. Notice when we did clean up the code, it's still a mess but it's better than it was and the refactored code suggests other improvements as well. Also, maybe the next time someone is writing code they might have learned something and maybe, just maybe, they will not make the same mistake again, or at least not as often. While not direclty related, here is a great article to read that relates to this idea of feedback: Secrets of a Mind-Gamer by Joshua Foer . One of the things discussed in this article is what it takes to improve a skill: failure. That is, we need to make mistakes and then learn from those mistakes. What happens when tools allow us to not learn from our mistakes? I'm concerned that powerful tools might loosen or remove the pain associated with failure, which will lead to stagnation. If we are not always striving to make new stupid mistakes, then we aren't really learning, right?