This is part of my blog post on TDD for Android demonstrating some tests we may write.

PokemonDetailViewModel.kt

class PokemonDetailViewModel {

    private var pokemon: Pokemon? = null

    fun setPokemon(pokemon: Pokemon) {
        val typeCount = pokemon.types.size
        if (typeCount <= 0 || typeCount > 2) {
            throw IllegalArgumentException("Pokemon has an invalid number of types.")
        }

        this.pokemon = pokemon
    }

    val pokemonName: String
        get() = this.pokemon?.name.orEmpty()

    val firstType: String
        get() = this.pokemon?.types?.first().orEmpty()

    val secondType: String?
        get() = this.pokemon?.types?.getOrNull(1)

    val showSecondType: Boolean
        get() = this.secondType != null
}

PokemonDetailViewModelTest.kt

class PokemonDetailViewModelTest {

    @Test
    fun uninitializedPokemonReturnsDefaultValues() {
        val viewModel = PokemonDetailViewModel()

        assertEquals("", viewModel.pokemonName)
        assertEquals("", viewModel.firstType)
        assertNull(viewModel.secondType)
        assertFalse(viewModel.showSecondType)
    }

    @Test
    fun pokemonWithNoTypesThrowsException() {
        val viewModel = PokemonDetailViewModel()

        val testPokemon = Pokemon(name = "Squirtle")

        try {
            viewModel.setPokemon(testPokemon)
            fail("IllegalArgumentException was expected.")
        } catch (e: IllegalArgumentException) {
            // TODO: This is fine, we wanted this.
        }
    }

    @Test
    fun pokemonWithOneTypeExposesOnlyThatType() {
        val viewModel = PokemonDetailViewModel()

        val testPokemon = Pokemon(
            name = "Squirtle",
            types = listOf("Water")
        )

        viewModel.setPokemon(testPokemon)
        assertEquals("Squirtle", viewModel.pokemonName)
        assertEquals("Water", viewModel.firstType)
        assertNull(viewModel.secondType)
        assertFalse(viewModel.showSecondType)
    }

    @Test
    fun pokemonWithTwoTypesExposesBothTypes() {
        val viewModel = PokemonDetailViewModel()

        val testPokemon = Pokemon(
            name = "Bulbasaur",
            types = listOf("Grass", "Poison")
        )

        viewModel.setPokemon(testPokemon)
        assertEquals("Bulbasaur", viewModel.pokemonName)
        assertEquals("Grass", viewModel.firstType)
        assertEquals("Poison", viewModel.secondType)
        assertTrue(viewModel.showSecondType)
    }

    @Test
    fun pokemonWithMoreThanTwoTypesThrowsException() {
        val viewModel = PokemonDetailViewModel()

        val testPokemon = Pokemon(
            name = "Squirtle",
            types = listOf("One", "Two", "Three")
        )

        try {
            viewModel.setPokemon(testPokemon)
            fail("IllegalArgumentException was expected.")
        } catch (e: IllegalArgumentException) {
            // TODO: This is fine, we wanted this.
        }
    }
}