Neither the new method nor the existing (which should be present for some time as described above) seem to be covered with unit tests.

That is unfortunate, because the appropriate class TokenRetrievalClientTest already exists and it would be nice to have it covered. I see two options:

1. You may need to mock the creation of RestTemplate & KerberosRestTemplate (could be done by moving the creation to a method that you override in the test) to write the unit test properly.
2. A half-measure would be just to mock the fetchToken methods in the class to check if those are receiving expected parameters. Not a complete unit test, but even this would be better than nothing if the option 1 cannot be reasonably done (I believe it should be).

Since you are already providing the mocked exchange templates, I assume you want the full solution. Good choice.

However, you do not need to override all the fetchToken and refreshAccessToken (also these should be rather tested, not overridden). I think all you need is this:

while updating the with

case class TokenRetrievalClient(host: String) {

  private[client] def createKerberosRestTemplate( /* opt params or two methods? */ ): KerberosRestTemplate = {
    new KerberosRestTemplate( /* opt params?*/)
  }

  private[client] def createRestTemplate(): RestTemplate = {
    new RestTemplate()
  }

// and wherever the `new RestTemplate()` or `new KerberosRestTemplate(...)` is used within `TokenRetrievalClient`, you replace it with the methods above.

This way, you only override the very minimal to get your exchange mocks in -- while the rest of the TokenRetrievalClient stays original as is correctly subjected to unit testing.