Welcome to the second lesson in our exploration of Test-Driven Development (TDD) using Scala 3, ScalaTest, and Mockito. In the previous lesson, we covered how to utilize dummies to isolate dependencies. This lesson will focus on learning about another type of test double — Stubs.

By the end of this lesson, you will understand what stubs are and how to implement them in your tests within a Scala environment, specifically for isolating dependencies like external services.

In testing, test doubles help us isolate parts of our application. We've previously discussed dummies; now, we will explore stubs, a more useful type of test double. Stubs provide predefined answers to method calls during testing. Unlike other test doubles, stubs do not track their usage, making them simpler yet powerful for certain scenarios.

Stubs are particularly useful when testing functions that rely on external services or complex dependencies. By simulating function outputs, stubs make tests faster and more predictable. Keep in mind that stubs focus on ensuring your application's logic functions as expected without verifying the correctness of external dependencies.

In Scala, Mockito can be used to create stubs. It allows us to set up return values that simulate how dependencies should behave in a controlled environment. This predictability isolates and tests your application's logic without relying on the behavior of external systems, which might be complex or introduce variability.

To illustrate the concept of stubs, we will create a WeatherAlertService using stubs in a test-driven development process.

We will build a WeatherAlertService that fetches data from a WeatherService. This service will issue alerts based on specific conditions. The external data source is impractical for testing, so we'll use stubbed data for our tests instead.

Create a new test file named WeatherAlertServiceSpec.scala with the following test setup:

import org.scalatest.funspec.AnyFunSpec
import org.scalatestplus.mockito.MockitoSugar
import org.mockito.Mockito._

class WeatherAlertServiceSpec extends AnyFunSpec with MockitoSugar:

  describe("WeatherAlertService"):

    // A simple manual stub
    class WeatherServiceStub extends IWeatherService:
      private var weatherData = WeatherData(20, "sunny")

      def setWeather(temperature: Int, conditions: String): Unit =
        weatherData = WeatherData(temperature, conditions)

      override def getCurrentWeather(location: String): WeatherData =
        weatherData

    it("should return heat warning when temperature is above 35"):
      // Arrange
      val weatherService = WeatherServiceStub()
      weatherService.setWeather(36, "sunny")
      val alertService = WeatherAlertService(weatherService)

      // Act
      val alert = alertService.shouldSendAlert("London")

      // Assert
      assert(alert == "Extreme heat warning. Stay hydrated!")

In this test:

• We create a hand-crafted stub WeatherServiceStub that implements a trait IWeatherService. This stub allows us to customize the weather data by setting predefined values for temperature and conditions.
• The stub’s getCurrentWeather method returns a WeatherData object with these predefined values, simulating expected weather conditions for the test scenario without using Mockito.
• The ScalaTest framework is used to validate whether the WeatherAlertService correctly interprets the weather data and returns the appropriate alert, specifically checking if it generates a heat warning when the temperature exceeds 35 degrees.