An example of using the public APIs to test

FriendReferral.kt

fun referFriend(userId: String, referral: Referral) {
  if (isReferralValid(referral)) {
      processReferral(userId)
  }
}

private fun isReferralValid(referral: Referral): Boolean {
  // Checks the validity of the referral
}

// 
private fun processReferral(userId: String) {
  // Processes the referral
}

TestingInternalDetails.kt

// The condition here is that the private method is made pubic or package-private using VisibleForTesting annotation
@Test
fun `is referral valid with new referral expected value is true`() {
  val friendReferral = getInstance()
  asserTrue(friendReferral.isReferralValid(...))
}

// The condition here is that the private method is made pubic or package-private using VisibleForTesting annotation
@Test
fun `process referral valid with new referral verify referral successful`() {
  val friendReferral = getInstance()
  friendReferral.processReferral("testUserId")
  assertThat(referralList.contains("testUserId")).isTrue()
}

TestingUsingPublicAPI.kt

@Test
fun `refer friend with new referral verify referral successful`() {
  val friendReferral = getInstance()
  friendReferral.referFriend("testUserId", getStaleReferral())
  assertThat(referralList.contains("testUserId")).isTrue()
}

@Test
fun `refer friend with stale referral verify referral failue`() {
  val friendReferral = getInstance()
  friendReferral.referFriend("testUserId", getStaleReferral())
  assertThat(referralList.contains("testUserId")).isFalse()
}