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abstract class Bird {
    ...
    abstract fun fly()
}

class Penguin: Bird() {
    ...
    override fun fly() {
        throw Exception("Can't fly!")
    }

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// domain
sealed class ErrorEntity {
  
    sealed class ApiError: ErrorEntity() {
        // .....
    }

    sealed class FileError: ErrorEntity() {
        
        object NotFound: FileError()

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abstract class OnOneClickListener(private val minClickInterval: Long = 500) : View.OnClickListener {

    private var lastClickedTime: Long? = null

    override fun onClick(v: View) {
        val currentTime = SystemClock.elapsedRealtime()
        if(lastClickedTime == null || currentTime - lastClickedTime!! > minClickInterval) {
            onOneClick(v)
        }
    }

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package ch.immoscout24.ImmoScout24

import androidx.lifecycle.Observer

/**
 * Used for testing [androidx.lifecycle.LiveData]
 *
 * @param <T> type of data
 */
@Suppress("unused")

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private val usecase: TestUseCase = Mockito.mock(TestUseCase::class.java)

private val viewModel = ScreenViewModel(usecase)

private lateinit var stateObserver: LiveDataTestObserver<ScreenState>

@Before
fun setup() {
    stateObserver = LiveDataTestObserver()
    viewModel.liveData.observeForever(stateObserver) // start observing live data

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class LiveDataTestObserver<T> : Observer<T> {

    private val values: MutableList<T> = ArrayList()

    override fun onChanged(value: T?) {
        // whenever new value is set, add it into value list
        value?.let { values.add(value) }
    }

    /**

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@Test
fun `click button - success - should show loading then success message`() {
    Mockito.`when`(usecase.execute()).thenReturn(Completable.complete())
    viewModel.clickButton()

    // todo how to verify the state was turned into loading before showing the success message?

    // verify last state
    viewModel.liveData.value!!.apply {
        assertFalse(isLoading)

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class ScreenViewModel: ViewModel() {

    private val mutableLiveData = MutableLiveData<Int>()

    val liveData: LiveData<Int> = mutableLiveData

    fun clickButton() {
        mutableLiveData.value = (mutableLiveData.value ?: 0) + 1
    }
}

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data class ScreenState(
    val isLoading: Boolean,
    val isButtonEnabled: Boolean,
    val message: String?
)

class ScreenViewModel(private val usecase: TestUseCase): ViewModel() {

    private val disposable = CompositeDisposable()

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class ComponentA
class ComponentB(val a: ComponentA)

class MyTest : KoinTest {

    // Lazy inject property
    val componentB : ComponentB by inject()

    @Test
    fun `should inject my components`() {