Mocking using mockito

build.gradle

{
  testImplementation 'junit:junit:4.13.2'
  androidTestImplementation 'androidx.test.ext:junit:1.1.3'
  implementation 'com.google.code.gson:gson:2.8.6'
  
  testImplementation 'org.mockito:mockito-core:4.0.0'
  testImplementation 'com.nhaarman.mockitokotlin2:mockito-kotlin:2.2.0'
}

User.kt

data class User(
    val id:Int,
    val name:String,
    val email:String,
    val password:String
)

enum class LOGIN_STATUS{
    INVALID_USER,
    INVALID_PASSWORD,
    UNKNOWN_ERROR,
    SUCCESS
}

UserRepository.kt

class UserRepository{
    val users = listof<user>(
        User(id:1,"John","[email protected]","John@123"),
        User(id:2,"Wein","[email protected]","Wein@123"),
        User(id:3,"Emily","[email protected]","Emily@123")
    )
    
    fun loginUser(email: String, password: String): LOGIN_STATUS{
    
      //Fetch User From DB
      val users = users.filter {user -> user.email == email}
      
      return if (users.size == 1){
          if(users[0].password == password){
            LOGIN_STATUS.SUCCESS
          }else{
            LOGIN_STATUS.INVALID_PASSWORD
          }
      }else{
            LOGIN_STATUS.INVALID_USER
      }
    }
}

UserService.kt

class UserService(private val userRepository: UserRepository){

  fun loginUser(email: String, password: String): String{
      val status = userRepository.loginUser(email,password)
      
      return when(status){
          INVALID_USER -> "User does not exist"
          INVALID_PASSWORD -> "Password is invalid"
          UNKNOWN_ERROR  -> "Unknown error occurred"
          SUCCESS -> "logged in successfully"
      }
  }
}

UserServiceTest.kt

class UserServiceTest {

  @Mock
  lateinit var userRepository: UserRepository
  
  @Before
  fun serup(){
    MockitoAnnotations.openMocks(testClass:this)
    Mockito.`when`(userRepository.loginUser(anyString(),anyString())).thenReturn(LOGIN_STATUS.SUCCESS)
  }
  
  @Test
  fun testUserService(){
    val sut = UserService(userRepository)
    val status = sut.loginUser(email:"[email protected]",password:"111111")
    Assert.assertEquals(expected:"Logged in successfully",status)
  }
  
  
}