In the software development process, testing is one of the most crucial elements. To reiterate the importance of testing, there is even a software development technique called TDD or Test-Driven Development. This technique promotes writing software requirements as tests as the initial step in developing a code. The idea here is simple. Your attention should be on testing first. When your organization uses TDD, you should first use testing to explain the behavior of the software and then you should verify it and spot any probable errors. Only after the testing is completed, you can proceed with the real implementation to meet the testing needs. Before you understand the principles of Test Driven Development, it is better to first understand the basics of this software development technique.

Test-Driven Development – The Meaning:

Shortly and popularly called TDD, this is nothing but a practice of development of software by initially writing tests and then moving forward with the least amount of code needed to get through the tests. In simple terms, when you use this technique, you will have to first write your requirements clearly. Thereafter, you should write codes that can help you meet those requirements.

In addition, the Test-Driven Development technique also motivates other good practices like writing functional code as and when possible. It even recommends the usage of dependency injection. The reason is that these practices will make your code simple to test. Even, this usage will be beneficial in making your code more reusable, while the testing will make the future refactors considerably less tiresome.

Know the Three Rules of TDD:

In this process of learning the principles of TDD, you should be aware of the three rules of this software development methodology:

1. If a unit test fails, you should write product code only to pass the unit test.

2. You should not write more unit tests. You should write only those numbers that you can manage if there is a failure.

3. You should not write more production codes as compared to what is required for passing a single failing unit test.

Steps in Test-Driven Development:

Before throwing some light on the TDD Agile principles, it is better to first understand the five steps involved in the Test-Driven Development flow:

1. The first thing you will have to do is to read and understand the bug or feature request from clients.

2. Once you understand the requirements thoroughly, translate them by creating a unit test. In case, you have a hot reloading set up, the unit test will fail when you run it. Can you judge the reason? Yes, it fails because no code has been implemented so far.

3. Now, write and implement the code that meets the requirements. Then, run all tests and now they should pass. If they do not pass yet, you will have to repeat the steps until they pass.

4. Once they pass, clean your code up. You can do it by refactoring.

5. Finally, rinse, lather and follow the steps again.

You can understand the process better with the picture below:

The workflow in the picture above is known as red-green-refactoring. It is the outcome of the status of the tests within a cycle.

• Here, the red phase denotes that the code is not working.
• The green phase is an indication that everything is working. However, they do not work optimally.
• The blue phase denotes that the tester is refactoring the code. Nevertheless, the tester is confident that his code is covered with tests. With this confidence, the tester has the power to change or improve the code as required.

What Makes TDD Easier and Faster?

The very principles of Test Driven Development make TDD not only easier but also quicker.

TDD has gained popularity as a useful technique for designing and developing software to make sure that the code base is tested sufficiently and the tests have spotted its behavior as well.

The most common complaint from users of TDD is that they face hardships with maintaining code. For instance, they face issues with refactoring, bug fixing, the addition of new features, etc. The problem they face is that there are too many tests and so they are forced to spend most of their working hours fixing tests.

At times this can take place as tests try to capture more behavior as compared to what they should. Indeed, the application design should be optimized to better define boundaries of authority that can be tested with ease. The best state to achieve is one where your tests capture the entire behavior of the software without any sacrifice on the code maintenance so that development teams can head quickly.

When you engage in software development using the TDD approach, you will be designing the software using the SOLID Design principles of TDD which will result in fewer overall tests that will be a position to better capture the behavior of the programs you develop.

SOLID Principles of Test Driven Development – What Does It Mean?

You might have heard bout Extreme Programming if you are in the software development domain. TDD is one of the key tenants of XP, which has gained immense popularity in the software development industry.

If you wish to know the principles of TDD, you should know about SOLID. It is nothing but the acronym of five key design principles that make a code simple to understand, test and maintain. Most importantly, these principles will make the code more flexible as well. The SOLID principles are listed below:

1. Single Responsibility

2. Open/Closed

3. Liskov Substitution

4. Interface Segregation

5. Dependency Injection

Let us gain some understanding of these TDD agile principles here:

1. Single Responsibility Principle:

Based on this principle, a class must include a sole responsibility. This principle will make each class very simple to test. From the perspective of Test-Driven Development, this principle makes it easier to explain the behavior of every class that uses tests.

The objective of this principle is to generate tests that spot the unique behavior of every class implemented. It will help with bringing down testing redundant behavior across different layers or parts of the software.

2. Open/Closed Principle:

According to this principle, the entities of the program being developed should be kept open for extensions. But, they should remain closed for modifications. This principle insists that developers should be able to add new features to a program without making any changes to the code that already exists.

From the point of view of TDD, we will have to just create unit tests for testing and spotting the behavior of the fresh feature. We need not have to add any new tests to test as the behavior remains unchanged for this class.

3. Liskov Substitution Principle:

Based on this principle, objects in the software should be replaceable with instances of their subtypes. But, this must be possible without making any changes to the correctness of the software. This principle is related to the open/closed principle discussed above. To be precise, it states that the parts of a subclass shall be replaceable with objects of its subclasses without breaking the application. This needs the objects of your subclasses to react in the same way as the parts of your superclass. This principle, from the point of view of TDD, denotes that the unit tests should be written only for the new feature. The reason is that this is no risk of breaking other parts of the software.

4. Interface Segregation Principle:

As against a single general-purpose interface, many client-specific interfaces are better as per this principle. According to this principle, the software should be divided by explaining a fresh interface as against adding a new technique to the existing interface.

5. Dependency Injection Principle:

According to this principle, high-level elements must never rely on low-level elements. High and low-level components should rely on abstractions. Abstractions should not rely on details. Rather, details must rely on abstractions.

Key Learning Points:

One of the important points to remember about TTD is its entire departure from traditional software development techniques. Yes, as against writing all codes initially and finally running the tests, the crucial goal of TDD is to let the tests do the job of driving the code structure. One of the interesting things about TDD is its iterative nature and continuous adoption of the red-green-refactor loop that aids businesses to spot any bugs or issues early in the process.

How Does TDD Fit Into Agile Development?

Agile development insists on regular feedback. In turn, the expected product can be developed with perfection. Due to this nature of Agile Development, it is also called Feedback Driven Development. The probability of the project needing to change during the development sprint cycle is more. To handle this and to develop products aligned with the changing requirements of clients, teams should get constant feedback for preventing dishing out applications that cannot be used. With TDD, it is possible to get this feedback early in the development cycle.

Further, the test-first approach of TDD aids with the mitigation of crucial hindrances that come in the way of quality delivery of the application being developed. With constant bug fixes, feedback and extra features, the system improves to make sure that everything works as per the expectations. TDD improves association between team members from both the QA and Development Teams and even with the client. Further, as the tests are created in advance, teams need not have to spend time recreating comprehensive test scripts. 

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