Welcome to the first lesson of our course on Test Driven Development (TDD) in Kotlin using JUnit and Mockito. TDD is an iterative software development process where tests are written before developing the actual functionality. This methodology emphasizes understanding and meeting the requirements first, which helps create reliable and maintainable code.

In this lesson, we'll introduce you to the essential elements of TDD, including the Red-Green-Refactor cycle, which forms the backbone of this practice. We'll be utilizing tools suited for Kotlin: JUnit, a widely used testing framework for unit testing, and Mockito for mock and behavior-driven development. These tools are excellent for defining and running tests in Kotlin. Let’s get started by exploring the core components of TDD with a practical example.

The TDD process begins with writing a test that fails, marking the "Red" phase. This step helps you define precisely what the code should achieve before creating the actual implementation. Let's write a test for a sum function that will add two numbers.

Create a file named CalculatorTest.kt in the tests directory:

This test script:

• Uses to denote a single test case.