Writing your first test

1. In ScalaTest, you define tests inside classes that extend a style class such as FlatSpec (though in practice you'd usually directly extend a base class defined for your project, which extends a ScalaTest style class):

import org.scalatest.FlatSpec

class FirstSpec extends FlatSpec {
  // tests go here...
}

2. Each test in a FlatSpec is composed of a sentence that specifies a bit of required behavior and a block of code that tests it. The sentence needs a subject, such as "A Stack"; a verb, either should, must, or can; and the rest of the sentence. Here's an example:

"A Stack" should "pop values in last-in-first-out order"

If you have multiple tests about the same subject, you can use it to refer to the previous subject:

it should "throw NoSuchElementException if an empty stack is popped"

After the sentence you put the word in followed by the body of the test in curly braces. Here's a complete example:

import collection.mutable.Stack
import org.scalatest._

class StackSpec extends FlatSpec {

  "A Stack" should "pop values in last-in-first-out order" in {
    val stack = new Stack[Int]
    stack.push(1)
    stack.push(2)
    assert(stack.pop() === 2)
    assert(stack.pop() === 1)
  }

  it should "throw NoSuchElementException if an empty stack is popped" in {
    val emptyStack = new Stack[String]
    assertThrows[NoSuchElementException] {
      emptyStack.pop()
    }
  }
}

3. Place this in a file called StackSpec.scala and compile it using this Jar file:

$ scalac -cp scalatest-app_2.12.4-3.0.5.jar StackSpec.scala

4. You can run it from the command line by invoking ScalaTest's simple Runner:

$ scala -cp scalatest-app_2.12.4-3.0.5.jar org.scalatest.run StackSpec
Run starting. Expected test count is: 2
StackSpec:
A Stack 
- should pop values in last-in-first-out order
- should throw NoSuchElementException if an empty stack is popped
Run completed in 96 milliseconds.
Total number of tests run: 2
Suites: completed 1, aborted 0
Tests: succeeded 2, failed 0, ignored 0, pending 0
All tests passed.

5. Or you can run it from the Scala interpreter using the ScalaTest shell:

$ scala -cp scalatest_2.12.4-3.0.5.jar

scala> import org.scalatest._
import org.scalatest._

scala> run(new StackSpec)
StackSpec:
A Stack 
- should pop values in last-in-first-out order
- should throw NoSuchElementException if an empty stack is popped

Next, learn about using assertions.