For now, we have only Java code in the small sample Maven project. We are ready to introduce a few lines of Scala to the same codebase in order to show how both languages seamlessly interoperate. Let's create a src/test/scala directory, next to the existing java/ directory, where we will put our following new CustomerScalaTest.scala class, which is a similar test to the one we already have under src/test/java:

package com.demo.sample

import org.junit._
import Assert._

class CustomerScalaTest {

  @Before
  def setUp: Unit = {
  }

  @After
  def tearDown: Unit = {
  }

  @Test
  def testGetCustomerId = {
    System.out.println("getCustomerId")
    val instance = new Customer()
    val expResult: Integer = null
    val result: Integer = instance.getCustomerId()
    assertEquals(expResult, result)
  }
}

If we run the tests again, that is, type >mvn clean test again, the class will just be ignored as it is not a .java source file.