A Student's Guide to Software Engineering Tools & Techniques »

Test-Driven Development

Author: Yash Chowdhary

Reviewers: Ang Ze Yu, Neil Brian, James Pang, Tejas Bhuwania

What is Test-Driven Development?

Typically, you write tests for production code you've already written. Test-driven development (a.k.a TDD) is a software development technique that emphasizes writing tests before writing production code.

Robert Martin describes the "Three Laws of TDD" as -

"First Law: You may not write production code until you have written a failing unit test.

Second Law: You may not write more of a unit test than is sufficient to fail, and not compiling is failing.

Third Law: You may not write more production code than is sufficient to pass the currently failing test.”

These 3 laws help to formulate a step-by-step process of practising test-driven development. Figure 1 below highlights one iteration of this general TDD process. In practice, several such iterations are required - sometimes even to get the same test or portion of the test to pass.


Figure 1. General TDD process. [1]

Kent Beck, creator of Xtreme Programming and advocate of TDD, states that the general development process of TDD is as follows [2]:

  1. Add a test

    In this phase of the cycle, all you need to focus on is writing tests. Understanding the product's/feature's requirements and specifications is crucial at this stage of the development cycle.

  2. Run the test

    In this phase, you check if the new test added fails. This is an important step as it shows that the new test does not pass without requiring new code, and also rules out false positives (i.e. the test passes when it shouldn't). Furthermore, the reason for its failure must be clear.

  3. Write production code

    Here is when you write code to pass the test. The sole aim at this point is to pass the test and to write production code restricted to the scope of the test. It is possible that the code may not be in its best form (style- and syntax-wise), but that's something you need not worry about at this stage.

  4. Re-run test(s)

    Re-running the test suite and ensuring the test passes not only helps in keeping the cycle running, but also in increasing your confidence in the code that you have just written. This is because it shows that the new code meets the test requirements and does not break any existing features/requirements.

  5. Refactor the code

    As and when you write more production code and test to make sure it meets all your business, product, and/or feature requirements, it's important that the code itself meets a certain standard. Improving code style and readability, employing design patterns, and reducing duplication of code are some of the many things you can work on.

Steps 1-5 outline one iteration of the TDD process. When you combine multiple iterations of this process, it can be thought of as a TDD cycle.

Benefits of Test-Driven Development

Some reasons why teams can choose to adopt the approach of test-driven development are:

Benefit 1: Improves Code Quality

TDD can lead to well-written code. This is a direct consequence of the cycle that was explained above. One of the best practices while following TDD is to focus on small units of code (methods, classes, modules, etc). This can lead to more modular, flexible code with looser coupling that can be scaled. In this way, the code unit targets a specific requirement or feature with just enough code to fail, and descriptive enough for a new comer to understand it [3].

Additionally, developers work on units of code that can be written and tested independently and integrated later. This can be achieved through mock frameworks, which help drive home the rationale of writing unit tests.

The tests aim to cover all possible branches and paths that the production code can take because no more code is written than is necessary. This potentially indicates that the software will high code coverage. Detecting bugs at early stages is easier when you're working with code that has high coverage [4].

Benefit 2: Focuses on Understanding Product Requirements

Test-driven development requires the developers to have a good understanding of the product or feature requirements before any sort of development can begin. As mentioned earlier, the first step in development is writing tests. Coming up with tests without a clear picture of the requirements or specifications can be dangerous as:

  1. you could end up testing something that doesn't need to be tested, or
  2. you could end up overlooking some requirements that must be tested, thus giving you a false sense of accomplishment when you write production code to pass the test

TDD thus helps in preventing teams from rushing into development and pushes them to prioritize the requirements-gathering step, forcing them to understand the requirement of whatever small thing which will be implemented, whether it is a user story, a method, class, etc.

Benefit 3: Increases Productivity

A study done by researchers in CMU found that programmers who wrote more tests tended to be more productive [5], and since employing TDD meant writing more tests, there is a correlation between productivity and TDD.

Test-driven development makes programmers think about the desired outcome of certain processes, features, and how other code can be integrated. Programmers will then think about coming up with failing tests and working their way up to writing code that passes the test. Employing test-driven development thus helps to get a better understanding of the design of a program [3:1].

As mentioned before, test-driven development can potentially result in software with high code coverage. This, in turn, can boost individual and team morale, confidence in the code, and productivity as a consequence.


Even with some of the attractive benefits of adopting TDD, there is no one-size-fits-all solution for project design management. Some of the limitations of TDD are:

Limitation 1: Tough to Incorporate into Legacy Projects

Legacy projects [6] are hard to maintain even without having to incorporate TDD into them. As these projects have a history of design and implementation choices made based on the requirements and/or the resources at the time, investing man hours in revamping or "modernizing" them is not always deemed as top priority.

When you bring TDD into the picture, things get more complex as failing tests are written at first with the assumption that production code will be written to ensure they pass.

Writing production code in legacy systems with the goal of passing a specific test is harder than it seems. For instance, there may be dependencies between components that need to be "broken" for them to be testable, which could result in several workarounds. This could potentially add on to the complexity of the project, making it counter-productive as the entire point of the refactoring step is to make the code more readable [7].

Limitation 2: High Maintenance

Maintaining test-suites becomes a part of the overall overhead of the project. Since TDD-style approaches rely heavily on tests, there is a need for the tests to be in proper shape in order to prevent a false sense of correctness.

TDD also has a steep learning curve. The entire process takes time to get accustomed to. Refactoring of code by using good design patterns comes with time, experience, and a lot of practice.

Limitation 3: Possibility of Over-Emphasizing Unit Tests at the Expense of Other Tests

Unit tests are generally much more in number than other sort of tests like integration, system, or interface tests. Developers run the risk of giving less importance to these classes of tests if there are a high number of passing unit tests [8].

Getting Started with Test-Driven Development

As explained above, test-driven development is a software design technique. While it does have an established and unelaborate framework, there are elements of the development cycle that -

  1. could be new to you (such as testing and mock frameworks), or
  2. you require practice in (such as refactoring code)

For those of you who'd like to get your hands dirty, here are a couple of tutorials on getting started with unit testing with Java - one using the TestNG framework, and another using the JUnit framework. They are both comprehensive tutorials, covering topics from setting up your environment for testing and writing basic tests to annotations.

'Test-Driven Development: By Example' by Kent Beck is a worthwhile read. Meant to inspire developers to embrace TDD, this book discusses the crux of the approach along with best practices, techniques and sample projects.

For those interested in diving deeper, Scott W. Ambler's 'Introduction to Test-Driven Development (TDD)' gives an in-depth understanding of the topic.


  1. Agile Data, Scott W. Ambler

  2. Beck, K. 2000. "Extreme Programming Explained"

  3. Effective TDD for Complex, Embedded Systems Whitepaper

  4. P. S. Kochhar, F. Thung and D. Lo, 2015. "Code coverage and test suite effectiveness: Empirical study with real bugs in large systems"

  5. Erdogmus, 2005. "On the Effectiveness of Test-first Approach to Programming"

  6. Legacy Code

  7. Smart, J.F. 2009. "An Introduction to Test-Driven Development with Legacy code"

  8. Problems with TDD