Kotlin find max temp city - single row definition

gistfile1.txt

  data class City(val temp:Double)

  class Utility {
    fun maxTemp(cities: List<City>?): City = cities?.maxBy { it.temp }?: throw IllegalArgumentException("param cities should not be null or empty!")
  }
  
  //extension funciton - maxBy is integrated in Kotlin by default
  fun List<City>.maxTemp(): City = this.maxBy { it.temp }?: throw IllegalArgumentException("param cities should not be null or empty!")

  @Test
  fun testFindMaxTempCity() {
    val cities = listOf(City(1.0),City(2.0),City(3.0))

    val maxTempCity = Utility().maxTemp(cities)

    assertThat(maxTempCity, Matchers.`is`(City(3.0)))
  }

  @Test(expected = IllegalArgumentException::class)
  fun findMaxTempCityInEmptyList() {
    val cities = listOf<City>()

    Utility().maxTemp(cities)
  }
  @Test(expected = IllegalArgumentException::class)
  fun findMaxTempCityNullList() {

    Utility().maxTemp(null)
  }