test.scala

trait Foo {
  def execute(): Unit
}

object Foo extends Foo {
  def execute(): Unit = println("foo")
}

trait Bar {
  val foo: Foo = Foo
}

object Main extends App {
  object FooMock extends Foo {
    def execute(): Unit = println("mock dayo")
  }

  val bar = new Bar {
    override val foo: Foo = FooMock // Mockに差し替え
  }

  bar.foo.execute()
}