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What is Unit Testing?

What really makes code untestable is hard-coded, impure factors that cannot be replaced, overridden, or abstracted away in some other way. Impurity is toxic: if method Foo depends on non-deterministic or side-effecting method Bar , then Foo becomes non-deterministic or side-effecting as well. Eventually, we may end up poisoning the entire codebase. However, impurity is inevitable; any real-life application must, at some point, read and manipulate state by interacting with the environment, databases, configuration files, web services, or other external systems.

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Static properties and fields or, simply put, global state, can complicate code comprehension and testability, by hiding the information required for a method to get its job done, by introducing non-determinism, or by promoting extensive usage of side effects. Functions that read or modify mutable global state are inherently impure. For example, it is hard to reason about the following code, which depends on a globally accessible property:.

Now , or Environment. MachineName ; they are read-only, but still non-deterministic. On the other hand, immutable and deterministic global state is totally OK. Constant values like Math. PI do not introduce any non-determinism, and, since their values cannot be changed, do not allow any side effects:.

Tips for writing great unit tests

Essentially, the Singleton pattern is just another form of the global state. Singletons promote obscure APIs that lie about real dependencies and introduce unnecessarily tight coupling between components. They also violate the Single Responsibility Principle because, in addition to their primary duties, they control their own initialization and lifecycle. Singletons can easily make unit tests order-dependent because they carry state around for the lifetime of the whole application or unit test suite.

Have a look at the following example:.


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In the example above, if a test for the cache-hit scenario runs first, it will add a new user to the cache, so a subsequent test of the cache-miss scenario may fail because it assumes that the cache is empty. Using Singletons is a bad practice that can and should be avoided in most cases; however, it is important to distinguish between Singleton as a design pattern, and a single instance of an object.

In the latter case, the responsibility of creating and maintaining a single instance lies with the application itself. This approach is absolutely correct, from both testability and API quality perspectives. Newing up an instance of an object in order to get some job done introduces the same problem as the Singleton anti-pattern: unclear APIs with hidden dependencies, tight coupling, and poor testability. For example, in order to test whether the following loop stops when a status code is returned, the developer should set up a test web server:.

However, sometimes new is absolutely harmless: for example, it is OK to create simple entity objects:.


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It is also OK to create a small, temporary object that does not produce any side effects, except to modify their own state, and then return the result based on that state. Static methods are another potential source of non-deterministic or side-effecting behavior. They can easily introduce tight coupling and make our code untestable. For example, to verify the behavior of the following method, unit tests must manipulate environment variables and read the console output stream to ensure that the appropriate data was printed:.

However, pure static functions are OK: any combination of them will still be a pure function. For example:. Obviously, writing testable code requires some discipline, concentration, and extra effort. But software development is a complex mental activity anyway, and we should always be careful, and avoid recklessly throwing together new code from the top of our heads. After all, the ultimate advantage of testable code is not only the testability itself, but the ability to easily understand, maintain and extend that code as well.

Unit testing is a method that instantiates a small part of our code and verifies its behavior independently from other parts of the project. For a unit test to be successful, the resulting behavior in all three phases must be in line with expectations. Integration testing focuses on testing and observing different software modules as a group. It is typically performed after unit testing is complete and precedes validation testing.

JUnit Tutorial - Java Unit Testing with Eclipse

Subscription implies consent to our privacy policy. Thank you! Check out your inbox to confirm your invite. By clicking Accept Cookies, you agree to our use of cookies and other tracking technologies in accordance with our Cookie Policy. Accept Cookies. Engineering All Blogs Icon Chevron. Filter by. View all results. Sergey Kolodiy. Sergey is a software engineer with extensive development experience in the.

What is Unit Testing? Integration Test Another important thing to consider is the difference between unit testing and integration testing. What Makes a Good Unit Test? Unit testing principles demand that a good test is: Easy to write. Testable and Untestable Code Some code is written in such a way that it is hard, or even impossible, to write a good unit test for it.

Now; if time. In its current form, this method suffers from several issues: It is tightly coupled to the concrete data source. AreEqual new DateTime , 12, 31, 23, 59, 59 , controller. Poisoning the Codebase with Side Effects Despite the fact that we solved the problems caused by the non-deterministic hidden input, and we were able to test certain functionality, the code or, at least, some of it is still untestable!

No matter how exactly light control is implemented, the SmartHomeController API suffers from these already-familiar issues: It is tightly coupled to the concrete implementation. Impurity and Testability Uncontrolled non-determinism and side effects are similar in their destructive effects on the codebase. The problem's not in your test suite.

It's in your code.

Part 1 Foundations

Understanding the basics. World-class articles, delivered weekly. Whether you are into the pricey first editions or interesting Seussiana ephemera, this is a good starting point for building a book collection based on Dr. Don't let a few small mistakes erode the value of your growing book collection. Review these tips to help preserve the beauty of your books!

What makes Biblio different? Facebook Instagram Twitter. Sign In Register Help Cart. Cart items. Toggle navigation. A couple concluding sections of the first chapter focus on using tests for design and employing behavior-driven development BDD. In this first chapter, the author also introduces his "The Law of Two Plateaus" to differentiate between using unit tests solely for quality assurance versus using them for design in addition to quality assurance. However, it does manage in 12 pages to meet the author's goal for it and the other two chapters Chapter 2 and Chapter 3 of Part 1 of providing a "shared context" for the remainder of the book.

Chapter 2 delves deeper into the question of "What makes a test 'good'? Koskela discusses in this chapter how virtues of regular source code are often valued virtues of test code. For example, he discusses that readable test code is maintainable test code and that appropriate structure plays a big role in making tests understandable. Koskela devotes a section of Chapter 2 to a smaller but significant issue I've seen repeatedly in writing and maintaining my own and others' unit tests: unit test methods that advertise testing something they don't really test perhaps because they are named poorly can be very costly.

I like what Koskela titles that section, "It's not good if it's testing the wrong things. Another section of the second chapter focuses on the principle that, for good tests, "independent tests run easily in solitude. I like his "litmus test for a project's test infrastructure" to satisfy the following scenario: "Can I check out a fresh copy from version control to a brand new computer I just unboxed, run a single command, lean back, and watch a full suite of automated tests run and pass?

In addition to pointing out some unit test dependency smells in this section, Kosekla also provides some specific approaches that might be taken to address these. One of the sections of the second chapter of Effective Unit Testing looks at why testing the wrong thing or even testing nothing at all "happy tests" are problematic. There is coverage of why tests "need to be repeatable" along with references to Java-specific examples of tests that introduce things outside of the testing developer's control into the tests. The last section of Chapter 2 not counting the "Summary" introduces test doubles , the subject of Chapter 3.

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Koskela defines test doubles as "an umbrella term for The third chapter is the concluding chapter and my favorite chapter of Part 1 "Foundations". Koskela outlines five reasons developers might use test doubles, including "the most fundamental of the reasons for employing a test double - to isolate the code you want to test from its surroundings.

He then describes each of the types of test doubles and compares their strengths and weaknesses. Koskela's section "Guidelines for Using Test Doubles" introduces his "logic and heuristics" for "picking the [test double] option that results in the most readable test".