Using ScalaTest with sbt

ScalaTest is one of the main testing libraries for Scala projects, and in this lesson you’ll see how to create a Scala project that uses ScalaTest. You’ll also be able to compile, test, and run the project with sbt.

Creating the project directory structure

As with the previous lesson, create an sbt project directory structure for a project named HelloScalaTest with the following commands:

mkdir HelloScalaTest
cd HelloScalaTest
mkdir -p src/{main,test}/{java,resources,scala}
mkdir lib project target

Creating the build.sbt file

Next, create a build.sbt file in the root directory of your project with these contents:

name := "HelloScalaTest"
version := "1.0"
scalaVersion := "2.13.15"

libraryDependencies +=
    "org.scalatest" %% "scalatest" % "3.2.11" % Test

The first three lines of this file are essentially the same as the first example, and the libraryDependencies lines tell sbt to include the dependencies (jar files) that are needed to run ScalaTest:

libraryDependencies +=
    "org.scalatest" %% "scalatest" % "3.2.11" % Test

The ScalaTest documentation has always been good, and you can always find the up to date information on what those lines should look like on the Installing ScalaTest page.

Create a Scala file

Next, create a Scala program that you can use to demonstrate ScalaTest. First, from the root directory of your project, create a directory under src/main/scala named simpletest:

$ mkdir src/main/scala/simpletest

Then, inside that directory, create a file named Hello.scala with these contents:

package simpletest

object Hello extends App {
    val p = new Person("Alvin Alexander")
    println(s"Hello ${p.name}")
}

class Person(var name: String)

There isn’t much that can go wrong with that source code, but it provides a simple way to demonstrate ScalaTest. At this point you can run your project with the sbt run command, where your output should look like this:

> sbt run

[warn] Executing in batch mode.
[warn]   For better performance, hit [ENTER] to switch to interactive mode, or
[warn]   consider launching sbt without any commands, or explicitly passing 'shell'
...
...
[info] compiling 1 Scala source to /Users/al/Projects/Scala/HelloScalaTest/target/scala-2.13/classes...
[info] running simpletest.Hello 
Hello Alvin Alexander
[success] Total time: 4 s

Now let’s create a ScalaTest file.

Your first ScalaTest tests

ScalaTest is very flexible, and there are a lot of different ways to write tests, but a simple way to get started is to write tests using the ScalaTest “FunSuite.” To get started, create a directory named simpletest under the src/test/scala directory, like this:

$ mkdir src/test/scala/simpletest

Next, create a file named HelloTests.scala in that directory with the following contents:

package simpletest

import org.scalatest.funsuite.AnyFunSuite

class HelloTests extends AnyFunSuite {

    // test 1
    test("the name is set correctly in constructor") {
        val p = new Person("Barney Rubble")
        assert(p.name == "Barney Rubble")
    }

    // test 2
    test("a Person's name can be changed") {
        val p = new Person("Chad Johnson")
        p.name = "Ochocinco"
        assert(p.name == "Ochocinco")
    }

}

This file demonstrates the ScalaTest FunSuite approach. A few important points:

• Your class should extend AnyFunSuite
• You create tests as shown, by giving each test a unique name
• At the end of each test you should call assert to test that a condition has been satisfied

Using ScalaTest like this is similar to JUnit, so if you’re coming to Scala from Java, hopefully this looks very familiar.

Now you can run these tests with the sbt test command. Skipping the first few lines of output, the result looks like this:

> sbt test
[info] set current project to HelloScalaTest (in build file:/Users/al/Projects/Scala/HelloScalaTest/)
[info] HelloTests:
[info] - the name is set correctly in constructor
[info] - a Person's name can be changed
[info] Run completed in 277 milliseconds.
[info] Total number of tests run: 2
[info] Suites: completed 1, aborted 0
[info] Tests: succeeded 2, failed 0, canceled 0, ignored 0, pending 0
[info] All tests passed.
[success] Total time: 1 s

TDD tests

This example demonstrates a Test-Driven Development (TDD) style of testing with ScalaTest. In the next lesson you’ll see how to write Behavior-Driven Development (BDD) tests with ScalaTest and sbt.

Keep the project you just created. You’ll use it again in the next lesson.