Welcome to the first lesson of our course on managing test doubles. In this lesson, we will explore the concept of dependencies in software testing and introduce you to the use of test doubles, starting with dummies, the simplest form of test doubles.

Dependencies are components or services that your software relies on to function, like databases, logging systems, or external APIs. However, when testing, these dependencies can introduce variability, making it hard to test your code's logic reliably. Test doubles allow you to replace these real dependencies with simpler objects that mimic their behavior. This ensures tests focus solely on your code's logic without interference from external systems. For instance, by isolating an email service’s logging component using a test double, you can test email-related functionality without generating actual log entries.

If your test suite interacts with a real email API, it might fail due to network issues or usage limits—problems unrelated to your code's logic. By using test doubles, you ensure that such failures don't affect your test outcomes, leading to more reliable and maintainable tests. The isolation is especially valuable when the dependency involves side effects—like writing to disk, accessing the network, or updating a database—because it ensures your tests don’t have unintended consequences. Additionally, it allows you to run tests in any environment without needing configuration for the actual external services.

During this course, we'll discuss five kinds of Test Doubles:

• Dummies: These are simple placeholders used to fulfill parameter requirements. They have no logic or behavior beyond satisfying an interface or method signature.
• Stubs: These provide predefined responses to specific calls during testing, allowing you to control the behavior of certain dependencies without implementing full functionality.
• Spies: These track information about interactions with dependencies, such as method calls and arguments, enabling you to verify behaviors indirectly.