Scalatest - how to test println

Question

Is there something in Scalatest that will allow me to test the output to the standard out via a println statement?

So far I've mainly been using FunSuite with ShouldMatchers.

e.g. how do we check the printed output of

object Hi {   def hello() {     println("hello world")   } } 

Answer 1

If you just want to redirect console output for a limited duration, use the withOut and withErr methods defined on Console:

val stream = new java.io.ByteArrayOutputStream() Console.withOut(stream) {   //all printlns in this block will be redirected   println("Fly me to the moon, let me play among the stars") } 

Answer 2

The usual way to test print statements on the console is to structure your program a bit differently so that you can intercept those statements. You can for example introduce an Output trait:

  trait Output {     def print(s: String) = Console.println(s)   }    class Hi extends Output {     def hello() = print("hello world")   } 

And in your tests you can define another trait MockOutput actually intercepting the calls:

  trait MockOutput extends Output {     var messages: Seq[String] = Seq()      override def print(s: String) = messages = messages :+ s   }     val hi = new Hi with MockOutput   hi.hello()   hi.messages should contain("hello world")