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25 Assertions and Tests

25.1 Assertions

  def above(that: Element): Element =
    val this1 = this widen that.width 
    val that1 = that widen this.width 
    assert(this1.width == that1.width)
    elem(this1.contents ++ that1.contents) 


  private def widen(w: Int): Element =
    if w <= width then
      this 
    else { 
      val left = elem(' ', (w - width) / 2, height) 
      var right = elem(' ', w - width - left.width, height) 
      left beside this beside right 
    } ensuring (w <= _.width)


  

25.2 Testing in Scala

// In file assertions-and-tests/ElementSuite.scala

  import org.scalatest.funsuite.AnyFunSuite
  import Element.elem

  class ElementSuite extends AnyFunSuite:

    test("elem result should have passed width") {
      val ele = elem('x', 2, 3)
      assert(ele.width == 2)
    }


  scala> (new ElementSuite).execute()
  ElementSuite:
  - elem result should have passed width


  

25.3 Informative failure reports

  scala> val width = 3
  width: Int = 3

  scala> assert(width == 2)
  org.scalatest.exceptions.TestFailedException:
      3 did not equal 2


  scala> assert(List(1, 2, 3).contains(4))
  org.scalatest.exceptions.TestFailedException:

    assert(List(1, 2, 3).contains(4))
           |    |  |  |  |        |
           |    1  2  3  false    4
           List(1, 2, 3)


  assertResult(2) {
    ele.width
  }


  assertThrows[IllegalArgumentException] {
    elem('x', -2, 3)
  }


  Expected IllegalArgumentException to be thrown,
    but NegativeArraySizeException was thrown.


  val caught =
    intercept[ArithmeticException] {
      1 / 0
    }

  assert(caught.getMessage == "/ by zero")


  

25.4 Tests as specifications

// In file assertions-and-tests/ElementSpec.scala

  import org.scalatest.flatspec.AnyFlatSpec
  import org.scalatest.matchers.should.Matchers
  import Element.elem

  class ElementSpec extends AnyFlatSpec, Matchers:

    "A UniformElement" should
        "have a width equal to the passed value" in {
      val ele = elem('x', 2, 3)
      ele.width should be (2)
    }

    it should "have a height equal to the passed value" in {
      val ele = elem('x', 2, 3)
      ele.height should be (3)
    }

    it should "throw an IAE if passed a negative width" in {
      an [IllegalArgumentException] should be thrownBy {
        elem('x', -2, 3)
      }
    }


  scala> (new ElementSpec).execute()
  A UniformElement
  - should have a width equal to the passed value
  - should have a height equal to the passed value
  - should throw an IAE if passed a negative width


result must be >= 0
map must contain key 'c'


Map('a' -> 1, 'b' -> 2) did not contain key 'c'


  import org.specs2.*
  import Element.elem

  object ElementSpecification extends Specification:
    "A UniformElement" should {
      "have a width equal to the passed value" in {
        val ele = elem('x', 2, 3)
        ele.width must be_==(2)
      }
      "have a height equal to the passed value" in {
        val ele = elem('x', 2, 3)
        ele.height must be_==(3)
      }
      "throw an IAE if passed a negative width" in {
        elem('x', -2, 3) must
          throwA[IllegalArgumentException]
      }
    }


// In file assertions-and-tests/TVSetSpec.scala

import org.scalatest.*
import org.scalatest.featurespec.AnyFeatureSpec

class TVSetSpec extends AnyFeatureSpec, GivenWhenThen:

  Feature("TV power button") {
    Scenario("User presses power button when TV is off") {
      Given("a TV set that is switched off")
      When("the power button is pressed")
      Then("the TV should switch on")
      pending
    }
  }


  

25.5 Property-based testing

// In file assertions-and-tests/ex2/ElementSpec.scala

  import org.scalatest.wordspec.AnyWordSpec
  import org.scalatestplus.scalacheck.ScalaCheckPropertyChecks
  import org.scalatest.matchers.must.Matchers.*
  import Element.elem

  class ElementSpec extends AnyWordSpec,
          ScalaCheckPropertyChecks:
    "elem result" must {
      "have passed width" in {
        forAll { (w: Int) =>
          whenever (w > 0) {
            elem('x', w % 100, 3).width must equal (w % 100)
          }
        }
      }
    } 


  whenever (w > 0) {
    elem('x', w % 100, 3).width must equal (w % 100)
  }


  

25.6 Organizing and running tests

name := "ThankYouReader!"

scalaVersion := "3.0.0"

libraryDependencies += "org.scalatest" %% "scalatest" %
    "3.2.9" % "test"


$ sbt
[info] welcome to sbt 1.5.2 (AdoptOpenJDK Java 1.8.0_262)
...
sbt:ThankYouReader!>


  

25.7 Conclusion