class Repository {
  fun getAll(): List<String> {
    // retrieve or create list of strings somehow
// return only entries containing letter "g"
class UseCase(private val repository: Repository) {
  fun execute(): List<String> {
    return repository.getAll().filter { it.contains("g") }
And following test method:
class UseCaseTest {
  @Test
  fun `it should return elements containing letter g`() {
    val mockRepository: Repository = mock {
      on { getAll() } doReturn listOf("kotlin", "testing", "blog")
    val useCase = UseCase(
      repository = mockRepository
    val result = useCase.execute()
    result.size shouldBe 2
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