Test-Driven Development in Java/

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Test Design with SOLID Principles

Test Design with SOLID Principles

Explore design tests using SOLID principles with a focus on the Single Responsibility Principle (SRP) as a modular approach.

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Design decisions

To write our first test, we ran through a number of design decisions. Let’s review that initial test code and list all the design decisions we had to make, as follows:

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@Test
public void oneIncorrectLetter() {
    var word = new Word("A");
    var score = word.guess("Z");
    assertThat( score.letter(0) ).isEqualTo(Letter.INCORRECT);
}

We decided on the following test writing flow:

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What to test?
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What to test?

These are all design decisions that our human minds must make. TDD leaves us very much hands-on when it comes to designing our code and deciding how it should be implemented. To be honest, we’re happy about that. Designing is rewarding, and TDD provides helpful scaffolding rather than a prescriptive approach. It acts as a guide to remind us to make these design decisions early.

It also provides a way to document these decisions as test code—nothing more, but equally, nothing less. It can be helpful to use techniques such as pair programming or mobbing (also known as ensemble programming) as we make these decisions—then, we add more experience and ideas to our solution. Working alone, we simply have to make the best decisions we can, based on our own experience.

SOLID principles

The critical point to get across here is that TDD does not and cannot make these decisions for us. We must make them. As such, it’s useful to have some guidelines to steer us toward better designs. A set of five design principles known as the SOLID principles is helpful. SOLID is an acronym for the following five principles:

  • SRPSingle Responsibility Principle

  • OCPOpen-Closed Principle ...