A Compendium of Software Design

No production code constants in tests

When writing tests, we might be tempted to use production code constants to remove duplication. This creates a conflict of interests by which a test is defined using part of the code that is supposed to test. Such a situation can lead to bugs going unnoticed like in the following example.

(In Kotlin, companion object is the way to define constants known at compile time, the equivalent of static in Java)

class Greeter {

  fun greet(name: String): String {
    return "$DEFAULT_GREETING $name"
  }

  companion object {
    const val DEFAULT_GREETING: String = "Hello"
  }
}

@Test
fun `greet by name`() {
  val greeter = Greeter()

  val result = greeter.greet("Andrea")

  assertEquals("${Greeter.DEFAULT_GREETING} Andrea", result)
}

The problem with the above code is that if by mistake we change the value of DEFAULT_GREETING from "Hello" to "Hellozxhgj" the test will still pass as it would be expecting "Hellozxhgj Andrea". To catch this kind of bugs is enough not to use the production constant DEFAULT_GREETING in the test like follows.

@Test
  fun `greet by name`() {
    val greeter = Greeter()

    val result = greeter.greet("Andrea")

    assertEquals("Hello Andrea", result)
  }

Teach me back

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