[FR]: Migrate from JUnit assertions to Kotlin's test framework

[SimonMarquis]

Is there an existing issue for this?

• I have searched the existing issues

Describe the problem

JUnit assertions have not been designed with Kotlin in mind.

Describe the solution

On the other hand, Kotlin's test framework make use of Kotlin's strenghts and we could benefit from it.

• better error messages by default

   Assert.assertTrue(item is TopicUiState.Success)
   // java.lang.AssertionError

  vs

   kotlin.test.assertIs<TopicUiState.Success>(item)
   // Expected value to be of type <com.google.samples.apps.nowinandroid.feature.topic.TopicUiState.Success>, actual <class com.google.samples.apps.nowinandroid.feature.topic.TopicUiState$Error>.

• better type inference

   // This code would produce an error because `getCurrentFollowedTopics` return type is `Set<String>?`
   kotlin.test.assertEquals(emptySet<Int>(), userDataRepository.getCurrentFollowedTopics())
   // Instead, let the type inference do it's job by omiting the type altogether
   assertEquals(emptySet(), userDataRepository.getCurrentFollowedTopics())

• smart cast

   val item = viewModel.topicUiState.value
   assertIs<TopicUiState.Success>(item)
   // Now item can be used as a TopicUiState.Success

Additional context

No response

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