Why "getSharedPreferences" return null in unit testing? - unit-testing

Question

My classes is written in Kotlin and here is my SharedPreferenceHandler
class SharedPreferenceHandler(sharedPrefs: SharedPreferences) {
companion object {
var mInstance: SharedPreferenceHandler = SharedPreferenceHandler(getPrefs())
private fun getPrefs(): SharedPreferences {
return Application.mInstance.getSharedPreferences(
"myApp", Context.MODE_PRIVATE)
}
fun getInstance(): SharedPreferenceHandler {
return mInstance
}
}
private var sharedPreferences = sharedPrefs
var accessToken: String?
get() = sharedPreferences.getString(SharedPreference.ACCESS_TOKEN.name, null)
set(token) = sharedPreferences.edit().putString(SharedPreference.ACCESS_TOKEN.name, token).apply()
}
Here is method called in presenter:
override fun reload(vm: ViewModel) {
super.updateViewModel(vm) {
//some stuffs
}
}
Here is my test method:
#Test
public void reload() {
when(SharedPreferenceHandler.Companion.getMInstance().getAccessToken()).thenReturn("234234234234234");
presenter.reload(viewModel);
}
In handler from super.updateViewModel(vm) I call "SharedPreferenceHandler.mInstance.accessToken!!)"
That is what is thrown:
Caused by: java.lang.IllegalStateException:
Application.mInstanc…m", Context.MODE_PRIVATE) must not be null
at
com.zuum.zuumapp.preferences.SharedPreferenceHandler$Companion.getPrefs(SharedPreferenceHandler.kt:18)
at
com.zuum.zuumapp.preferences.SharedPreferenceHandler$Companion.access$getPrefs(SharedPreferenceHandler.kt:14)
at
com.zuum.zuumapp.preferences.SharedPreferenceHandler.(SharedPreferenceHandler.kt:15)
I wanna to get accessToken by calling " SharedPreferenceHandler.mInstance.accessToken!!" in my test class.
Is possible to get that in my test method?

Answer 1

You can't use Android SharedPreferences in unit test, but you can mock your method call by this:
Mockito.`when`(SharedPreferenceHandler.mInstance.accessToken).thenReturn("token")
And return what you need.

Answer 2

You should not test your code this way. You should create an interface for class you want to mock:
interface MySharedPreferences {
fun getAccessToken(): String
}
Let your SharedPreferencesHandler implements this interface. Then in your presenter (or other class you want to test) inject dependencies (f.e. by constructor or framework like Dagger/Kodein) into your object. Then there is possibility to easy mock this interface. I assume in #Before you create class you test - and then just pass as param your mocked SharedPreferencesHandler.
Testing things with static dependencies is possible, but is but tricky (and a lot of people consider static dependencies as anti-pattern). How to do it is described here: How to android unit test and mock a static method
Example:
class MyPresenter(val sp: MySharedPreferences) {
/* some code here */
fun validateToken() {
if (sp.getAccessToken() == "") throw new Exception()
}
}
Like you see sp is injected into this class as parameter. Normally you don't create views/presenters etc. directly in code but by DI framework (like Dagger or Kodein). Anyway, static dependencies are not easy testable. Injected interface-dependencies can be mocked, and you operating not on object, but on behaviors (so it's bigger level of abstraction). So, now in your test all you have to do is:
class MyTest() {
#Mock lateinit var sharedPreferencesMock: MySharedPreferences
lateinit var instance: MyPresenter
#Before
fun setUp() {
instance = MyPresenter(sharedPreferencesMock)
}
#Test
fun testSomething() {
`when`(sharedPreferencesMock.getAccessToken()).thenReturn("myAccessToken")
/* here is your test body */
}
}