Simple FlatSpec

// build.sbt:
libraryDependencies ++= Seq(
  "org.scalatest"     %% "scalatest"            % "3.0.5"         % Test,
  "org.mockito"       %% "mockito-scala"        % "1.0.9"         % Test
)

// ClassA.scala
class ClassA(dependentClass: DependentClass) {
  private lazy val task = Task(0)
  private lazy val user = User(None, "", "", None)

  def getCurrentTaskDuration: Long = {
    dependentClass.getDurationMsec(task, user)
  }

  def sayHello: String = "hello"
}

// DependentClass.scala:
class DependentClass {
  def getDurationMsec(task: Task, user: User): Long = ???
}



// Spec:
import org.scalatest.FlatSpec
import org.scalatest._
import org.scalatest.mockito.MockitoSugar
import org.mockito.Mockito._
import org.mockito.ArgumentMatchers._

import my.package._

class MySpec extends FlatSpec with Matchers with MockitoSugar {
  behavior of "ClassA"

  val dependentClass: DependentClass = mock[DependentClass]
  val classA = new ClassA(dependentClass)

  when(dependentClass.getDurationMsec(any[Task], any[User])).thenReturn(1000)

  "A classA" should "return 1000 for a given task" in {
    val result = classA.getCurrentTaskDuration
    result shouldBe 1000
  }

  it should "say hello" in {
    val result = classA.sayHello
    result shouldBe "hello"
  }
}



// Output:
[info] MySpec:
[info] A classA
[info] - should return 1000 for a given task
[info] - should say hello
[info] ScalaTest
[info] Run completed in 2 seconds, 214 milliseconds.
[info] Total number of tests run: 2
[info] Suites: completed 1, aborted 0
[info] Tests: succeeded 2, failed 0, canceled 0, ignored 0, pending 0
[info] All tests passed.