MinimalDoFnSpec.scala

import org.apache.beam.sdk.transforms.DoFn.ProcessElement
import org.apache.beam.sdk.transforms.{DoFn, DoFnTester}
import org.scalatest.{FlatSpec, Matchers}


class MyDoFn extends DoFn[Integer, Integer] {
  @ProcessElement
  def processElement(c: ProcessContext): Unit = {
    c.output(c.element * 2)
  }
}

class MinimalDoFnSpec extends FlatSpec with Matchers {
  val tester: DoFnTester[Integer, Integer] = DoFnTester.of(new MyDoFn())

  "MyDoFn" should " multiply all elements by two" in {
    val actualOutput = tester.processBundle(1, 2, 3)
    val expectedOutput = Seq(2, 4, 6)

    actualOutput should contain only (expectedOutput: _*)
  }
}

For Testing DoFn with ScalaTest:

  "org.scalatest" %% "scalatest" % scalatestVersion % Test,