Either[X, Y] is an instance that contains either an instance of X or an instance of Y but not both. We call these subtypes left and right of Either. Creating an Either is trivial. But it's very powerful sometimes to use it in your program:

package com.chapter3.ScalaFP
import java.net.URL
import scala.io.Source
object Either {
  def getData(dataURL: URL): Either[String, Source] =
    if (dataURL.getHost.contains("xxx"))
      Left("Requested URL is blocked or prohibited!")
    else
      Right(Source.fromURL(dataURL))      
  def main(args: Array[String]) {
      val either1 = getData(new URL("http://www.xxx.com"))    
      println(either1)      
      val either2 = getData(new URL("http://www.google.com"))    
      println(either2)
  }
}

Now, if we pass any arbitrary URL that doesn't contain xxx then we will get a Scala.io.Source wrapped in a Right subtype. If the URL contains xxx, then we will get a String wrapped in a Left subtype. To make the preceding statement clearer, let's see the output of the preceding code segment:

Left(Requested URL is blocked or prohibited!) Right(non-empty iterator)

Next, we will explore another interesting feature of Scala called Future that is used to execute tasks in a non-blocking way. This is also a better way to handle the results when they finish.