How to Reuse Tests in Kotlin with Interface Default Methods

Writing test code can be tedious and time-consuming, especially when we need to test multiple implementations of the same interface. Fortunately, Kotlin provides a powerful feature called "interface default methods" that can help us to reuse our test code for multiple implementations.

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Let’s imagine, that we have an implementation we want to test

interface StringCalculator {

    fun lengthOf(string: String): Int

    fun upperCaseOf(string: String): String
}

class SlowStringCalculator : StringCalculator {

    override fun lengthOf(string: String): Int {
        sleep(100)
        return string.length
    }

    override fun upperCaseOf(string: String): String {
        sleep(100)
        return string.toUpperCase()
    }
}

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We added test interfaces with default methods to test our implementation

interface FirstExamplesUsingString : HasTestCalculator {

    @Test
    fun `string length is correct`() {
        assertEquals(stringCalculator.lengthOf("ab"), 2)
    }
}

interface SecondExamplesUsingString : HasTestCalculator {

    @Test
    fun `string uppercase is correct`() {
        assertEquals(stringCalculator.upperCaseOf("abc"), "ABC")
    }
}

Now we can compose a class from our interfaces and their default methods

class SlowStringCalculatorTests : SecondExamplesUsingString, FirstExamplesUsingString {
    override val stringCalculator = SlowStringCalculator()
}

✅ string uppercase is correct()

✅ string length is correct()

But after some time we added a new(faster) implementation of our StringCalculator

class FastStringCalculator : StringCalculator {

    override fun lengthOf(string: String): Int {
        return string.length
    }

    override fun upperCaseOf(string: String): String {
        return string.toUpperCase()
    }
}

And now we can reuse our test to apply them to our new implementation (and add more)

class FastStringCalculatorTests : SecondExamplesUsingString, FirstExamplesUsingString {
    override val stringCalculator = FastStringCalculator()

    @Test
    fun `another test for fast calculator`() {
        val startTime = currentTimeMillis()
        stringCalculator.upperCaseOf("abc")
        val endTime = currentTimeMillis()
        assertTrue(endTime - startTime < 100)
    }
}

✅ another test for fast calculator()