Solving java.util.NoSuchElementException: Flow is empty in Kotlin Unit Testing

---

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---

Understanding the Problem

In the context of unit testing for a repository that fetches weather information, you might want to simulate different scenarios, including failures.

Here’s a brief overview of the involved classes in your code:

AppRepository: Responsible for fetching weather data and mapping its results.

NetworkHelper: Simulates network calls to get weather information.

WeatherMapper: Maps raw data to a Weather object.

The core of the issue arises from your test case setup. Specifically, when you try to test a failure by returning an error wrapped in a Flow, you forget to emit that error.

Example Error in Your Test

In your test, the configuration looked like this:

[[See Video to Reveal this Text or Code Snippet]]

In this code, the Flow does not emit any value; hence it is empty. When you call single() on it, you get the NoSuchElementException.

The Solution

To resolve this issue, you need to ensure that your Flow actually emits a value. Below are the steps to fix your test case:

Step-by-step Fix

Updating the Flow to Emit a Value:
Modify your when clause to utilize the emit function correctly. The corrected code should look like this:

[[See Video to Reveal this Text or Code Snippet]]

Verifying the Result:
Now that your Flow will emit the error result, your test case can proceed to fetch this value using single() without raising an exception:

[[See Video to Reveal this Text or Code Snippet]]

Testing with Flow

When writing tests that involve Flow, keep the following in mind:

Make Sure to Emit Values: Always use the emit() function within the flow builder.

Understand Flow Behavior: Flow is asynchronous and can be cold; ensure that it is set up to emit values during the test.

Use Coroutine Test Utilities: Leverage testing utilities, such as TestCoroutineDispatcher, to handle coroutine execution in your tests effectively.

Conclusion

By following the outlined steps and keeping the tips in mind, you'll be better equipped to handle Flow in your Kotlin applications and write more robust unit tests. Happy coding!