As the book's title implies, the main tool this is all about is JUnit (http://www.junit.org). It is probably the most popular testing framework for developers within the Java world. Its first version was written by Kent Beck and Eric Gamma on a flight from Zurich to OOPSLA 1997 in Atlanta, [FOWL06]. Since then, it has evolved by adapting to changing language constructs, and quite a few supplemental libraries have emerged.

Java IDEs provide a UI and build path entries to compile, launch, and evaluate JUnit tests. Build tools, such as Ant, Maven, and Gradle, support test integration out of the box. When it comes to IDEs, the example screenshots in this book are captured using Eclipse (http://www.eclipse.org/). However, we do not rely on any Eclipse-specific features, which should make it easy to reproduce the results in your favorite IDE too.

In general, we use Maven (https://maven.apache.org/) for dependency management of the libraries mentioned next, which means that they can be retrieved from the Maven Central Repository (http://search.maven.org/). But if you clone the book's GitHub repository (https://github.com/fappel/Testing-with-JUnit), you will find a separate folder for each chapter, providing a complete project configuration with all dependencies and sources. This means navigating to this directory and using the 'mvn test' Maven command should enable you to compile and run the given examples easily. Let's finish this section with an introduction of the more important utilities we'll be using in the course of the book.

Chapter 3, Developing Independently Testable Units, covers the sense and purpose of the various test double patterns. It is no wonder that there are tools that simplify test double creation significantly. Usually, they are summarized under the term mock frameworks. The examples are based on Mockito (http://mockito.org), which suits very well to building up clean and readable test structures.

There are several libraries that claim to improve your daily testing work. Chapter 5, Using Runners for Particular Testing Purposes, will introduce JUnitParams (http://pragmatists.github.io/JUnitParams/) and Burst (https://github.com/square/burst) as alternatives to writing parameterized tests. Chapter 7, Improving Readability with Custom Assertions, will compare the two verification tools Hamcrest (http://hamcrest.org/) and AssertJ (http://assertj.org).

Automated tests are only valuable if they are executed often. Because of this, they are usually an inherent part of each project's continuous integration build. Hence, Chapter 8, Running Tests Automatically within a CI Build, will show how to create a basic build with Maven and introduce the value of code coverage reports with JaCoCo (http://www.eclemma.org/jacoco/).

In the beginning, one of the more profane-looking questions you have to agree upon within your team is where to put the test code. The usual convention is to keep unit tests in classes with the same name as the class under test, but post- or prefixed with an extension Test or such like. Thus, a test case for the Foo class might be named FooTest.

Based on the description of Hunt/Thomas, [HUTH03], of different project structuring types, the simplest approach would be to put our test into the same directory where the production code resides, as shown in the following diagram:

A single-source tree with the same package

We usually don't want to break the encapsulation of our classes for testing purposes, which shouldn't be necessary in most cases anyway. But as always, there are exceptions to the rule, and before leaving a functionality untested, it's probably better to open up the visibility a bit. The preceding code organization provides the advantage that, in such rare cases, one can make use of the package member access the Java language offers.

Members or methods without visibility modifiers, such as public, protected, and private, are only accessible from classes within the same package. A test case that resides in the same package can use such members, while encapsulation still shields them from classes outside the package, even if such classes would extend the type under test.

Unfortunately, putting tests into the same directory as the production code has a great disadvantage too. When packages grow, the test cases are perceived soon as clutter and lead to confusion when looking at the package's content. Because of this, another possibility is to have particular test subpackages, as shown here:

Single-source tree with a separate test package

However, using this structure, we give up the package member access. But how can we achieve a better separation of production and testing code without loosing this capability? The answer is to introduce a parallel source tree for test classes, as shown here:

A parallel-source tree

To make this work, it is important that the root of both trees are part of the compiler's CLASSPATH settings. Luckily, you usually do not have to put much effort in this organization as it is the most common one and gets set up automatically, for example, if you use Maven archetypes to create your projects. Examples in this book assume this structure.

Last but not least, it is possible to enhance the parallel tree concept even further. A far-reaching separation can be achieved by putting tests in their own source code project. The advantage of this strategy is the ability to use different compiler error/warning settings for test and production code. This is useful, for example, if you decide to avoid auto-boxing in your components but feel it would make test code overly verbose when working with primitives. With project-specific settings, you can have hard compiler errors in production code without having the same restriction in tests.

Parallel-source tree with separate test project

Whatever organization style you may choose, make sure that all team members use the same one. It will be very confusing and hardly maintainable, if the different concepts get mixed up. Now that the preliminaries are done, we are ready for action.

Let's assume that we have to write a simple timeline component as it is known from the various social networks, such as Twitter, Google+, Facebook, and the like. To make things a bit more interesting, the application has to run on different platforms (desktop, browser, and mobile) and allow the display of content from arbitrary sources. The wireframe in the following image gives an impression of the individual functional requirements of our timeline:

Timeline wireframe

The header contains a label indicating the source of the items displayed in the list under it. It also notifies the user if newer entries are available and allows the them to fetch and insert them at the top.

The list section is a sequence of chronologically ordered items, which can be browsed by a scrollbar. The component should allow us to load its entries page-wise. This means that it shows a maximum of, let's say, ten entries. If scrolling reaches the last one, the next ten items can be fetched from the provider. The newly loaded entries are added and the scrollbar is adjusted accordingly. To keep things in scope, a push button for manual fetching will be sufficient here.

An item type, in turn, comprises several text or image attributes that compose an entry's content. Note that the timestamp is considered mandatory as it is needed for chronological ordering. Apart from that, the depiction should be undetermined by the component itself and depend on the type of the underlying information source.

This means that a Twitter feed probably provides a different information structure than the commits of a branch in a Git repository. The following image shows what the running applications will look like. The JUnit items shown are commits taken from the master branch of the tool's project repository at GitHub.

Given the application description, it is important to note that the following chapters will focus on the unit testing aspects of the development process to keep the book on target. But this immediately raises the question: what exactly is a unit test?

Unit tests written with JUnit are grouped by plain Java classes, each of which is called a test case. A single test case specifies the behavior of a low-level component normally represented by a class. Following the metaphor of the accompanying specification, we can begin the development of our timeline example as follows:

public class TimelineTest {
}

The test class expresses the intent to develop the a component Timeline, which Meszaros, [MESZ07], would denote as system under test (SUT). And applying a common naming pattern, the component's name is complemented by the suffix Test. But what is the next logical step? What should be tested first? And how do we create an executable test anyway?

Usually, it is a good idea to start with the happy path, which is the normal path of execution and, ideally, the general business use case. Consider that we expect fetch-count to be an attribute of our timeline component. The value configures how many items will be fetched at once from an item source. To keep the first example simple, we will ignore the actual item loading for now and regard only the component's state change that is involved.

An executable JUnit test is a public, nonstatic method that gets annotated with @Test and takes no parameters. Summarizing all this information, the next step could be a method stub that names a functionality of our component we want to test. In our case, this functionality could be the ability to set the fetch-count to a certain amount:

public class TimelineTest {
  @Test
  public void setFetchCount() {
  }
}

This is still not much, but it is actually sufficient to run the test for the first time. JUnit test executions can be launched from the command line or a particular UI. But for the scope of this book, let's assume we have IDE integration available. Within Eclipse, the result would look like the next image.

The green progress bar signals that the test run did not recognize any problems, which is not a big surprise as we have not verified anything yet. But remember that we have already done some useful considerations that help us to populate our first test easily:

It looks reasonable so far, but how can we assure that a test run is denoted as a failure if the actual value returned by getFetchCount does not match the input used with setFetchCount? For this purpose, JUnit offers the org.junit.Assert class, which provides a set of static methods to help developers to write so-called self-checking tests.

The green bar after a successful launch

The methods prefixed with assert are meant to check a certain condition, throwing an java.lang.AssertionError on a negative evaluation. Such errors are picked up by the tool's runtime and mark the test as failed in the resulting report. To assert that two values or objects are equal, we can use Assert.assertEquals. As it is very common to use static imports for assertion method calls, the getFetchCount test can be completed like this:

@Test
public void setFetchCount() {
  Timeline timeline = new Timeline();
  int expected = 5;

  timeline.setFetchCount( expected );
  int actual = timeline.getFetchCount();

  assertEquals( expected, actual );
}

The built-in mechanism of JUnit, which is often considered somewhat dated, isn't the only possibility to express test verifications. But to avoid information flooding, we will stick to it for now and postpone a thorough discussion of the pros and cons of alternatives to Chapter 7, Improving Readability with Custom Assertions.

Looking at our first test, you can recognize that it specifies a behavior of the SUT, which does not even exist yet. And by the way, this also means that the test class does not compile anymore. So, the next step is to create a skeleton of our component to solve this problem:

public class Timeline {

  public void setFetchCount( int fetchCount ) {
  }

  public int getFetchCount() {
    return 0;
  }
}

Well, the excitement gets nearly unbearable. What will happen if we run our test against the newly created component?

Now the test run leads to a failure with a red progress bar due to the insufficient implementation of the timeline component, as shown in the next image. The execution report shows how many tests were run in total, how many of those terminated with errors, and how many failed due to unmet assertions.

A stack trace for each error/failure helps to identify and understand the problem's cause. AssertionError raised by a verification call of a test provides an explaining message, which is shown in the first line of the trace. In our example, this message tells us that the expected value did not meet the actual value returned by getFetchCount.

A test terminated by an Exception indicates an arbitrary programming mistake beyond the test's assertion statements. A simple example of this can be access to an uninitialized variable, which subsequently terminates test execution with NullPointerException. JUnit follows the all or nothing principle. This means that if an execution involves more then one test, which is usually the case, a single problem marks the whole suite as failed.

The red bar after test failure

The UI reflects this by painting the progress bar red. You would now wonder whether we shouldn't have completed our component's functionality first. The implementation seems easy enough, and at least, we wouldn't have ended up with the red bar. But the next section explains why starting with a failing test is crucial for a clean test-first approach.

Writing tests before the production code even exists might look strange to a newbie, but there are actually good reasons to do so. First of all, writing tests after the fact (meaning first code, then test) is no fun at all. Well, that sounds like a hell of a reason because, if you gotta do what you gotta do, [FUTU99], what's the difference whether you do it first or last?

The difference is in the motivation to do it right! Once you are done with the fun part, it is all too human to get rid of the annoying duties as fast and as sloppily as one can get through. You are probably reading this because you are interested in improving things. So, ask yourself how effective tests will be if they are written just for justification or to silence the conscience.

Even if you are disciplined and motivated to do your after the fact tests right, there will be more holes in the test coverage compared to the test-first approach. This is because the class under test was not designed for testing. Most of the time, it will take costly steps to decompose a component written from scratch into separate concerns that can be tested easily. And if these steps are considered too expensive, testing will be omitted. But isn't it a bad thing to change a design for testing purposes?

The point is that writing your tests first supports proper separation implicitly. Every time your test setup feels overly complicated, you are about to put too much functionality in your component. In this case, you should reconsider your class-level design and split it up into smaller pieces. Following this practice consequently leads to a healthy design on the class level out of the box.

Although this book is not about how to write tests first or test-driven development (TDD) as it is usually called, it follows this principle while developing the example application. But as the focus will be on getting unit tests right and not on the implementation aspects of the components, here come a few words about the work paradigm of TDD for better understanding.

The procedure is simple. Once you have picked your first work unit, write a test, make it run, and last, make it right, [BECK03]. After you're done, start it all over again with the next piece of functionality. This is exactly what we have done until now with our first test. We've decided about a small feature to implement. So, we wrote a test that specifies the intended behavior and invented a kind of programming interface that would match the use case.

When we feel confident with the outcome, it is about time to fix the compile errors and create a basic implementation stub to be able to execute the test. This way, the test is the first client of the freshly created component, and we will have the earliest possible feedback on how using it in programs will look. However, it is important that the first test run fails to ensure that the verification conditions were not met by accident.

The make it run step is about fixing the failing test as quickly as possible. This goal outweighs everything else now. We are even allowed to commit programming sins we usually try to avoid. If this feels a bit outlandish, think of it like this: if you want to write clean code that works (Ron Jeffries, [BECK03]) ensure that it works first and then take your time and clean it up second. This has the advantage that you know the specification can be met without wasting time in writing pretty code that will never work.

Last but not least, make it right. Once your component behaves as specified, ascertain that your production and test code follow the best programming standards you can think of. While overhauling your code, repeatedly executing the tests ensures that the behavior is kept intact. Changing code without changing its behavior is called refactoring.

In the overall image, we started with a failing test and a red bar, fixed the test, made the bar green again, and, finally, cleaned up the implementation during a last refactor step. As this pattern gets repeated over and over again in TDD, it is known as the red/green/refactor mantra.

So, always remember folks: keep the bar green to keep the code clean.