If testing is hard, inject what you need to verify

When you have a hard time testing something, the solution is usually to inject the thing you would like to verify. Suppose we are starting to implement a Car class that stores passengers by their name.

class Car {
  private val passengers: MutableSet<String> = HashSet()
    
  fun storePassenger(name: String) {
    passengers.add(name)
  }
}

How to test that the method storePassenger(name: String) stores a name into the passengers set? The typical solution is to define another method in Car to check if it contains a passenger.

class Car {
  private val passengers: MutableSet<String> = HashSet()
    
  fun storePassenger(name: String) {
    passengers.add(name)
  }
       
  fun containsPassenger(name: String): Boolean { 
    return passengers.contains(name)
  }
}

So we can write the following test

@Test
fun `stores the names of the passengers`() {
  val car = Car()
  
  car.storePassenger("Andrea")

  assertTrue(passengers.containsPassenger("Andrea"))
}

However, this is already a disappointment because we are forced to write a public method just for the sake of testing. Moreover, what if by specifications we must prevent any other code to query Car about its passengers? The solution can be to inject the passengers set a construction time.

class Car(private val passengers: MutableSet<String>) {

  fun storePassenger(name: String) {
    passengers.add(name)
  }
}

Now we can test like follows.

@Test
fun `stores the names of the passengers`() {
  val passengers = HashSet()
  val car = Car(passengers)
  
  car.storePassenger("Andrea")

  assertTrue(passengers.contains("Andrea"))
}

By injecting the passengers set we can get rid of the useless containsPassenger method. Furthermore, we have now the opportunity to make our code more modular, making Car independent of the data structure used to store the passengers. For this, we can use MutableCollection<String> instead of MutableSet<String>

class Car(private val passengers: MutableCollection<String>) {

  fun storePassenger(name: String) {
    passengers.add(name)
  }
}

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I really appreciate any feedback about the book and my current understanding of software design.

A Compendium of Software Design

If testing is hard, inject what you need to verify

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Teach me back