Scala Tutorial - Scala Option

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Option lets us express null value explicitly without the null "hack".

Option is an abstract class and its two concrete subclasses are Some, for when we have a value, and None, when we don't.

Example

You can see Option, Some, and None in action in the following example, where we create a map of state capitals in the United States:

object Main {
  def main(args: Array[String]) {
     val stateCapitals = Map( 
       "Alabama" -> "Montgomery", 
       "Alaska"  -> "Juneau", 
       "Wyoming" -> "Cheyenne") 

     println( "Get the capitals wrapped in Options:" ) 
     println( "Alabama: " + stateCapitals.get("Alabama") ) 
     println( "Wyoming: " + stateCapitals.get("Wyoming") ) 
     println( "Unknown: " + stateCapitals.get("Unknown") ) 

     println( "Get the capitals themselves out of the Options:" ) 
     println( "Alabama: " + stateCapitals.get("Alabama").get ) 
     println( "Wyoming: " + stateCapitals.get("Wyoming").getOrElse("Oops!") ) 
     println( "Unknown: " + stateCapitals.get("Unknown").getOrElse("Oops2!") ) 

  }
}

Note

The Map.get method returns an Option[T], where T is String in this case.

By returning an Option, we can't "forget" that we have to verify that something was returned.

If the Option is a Some, Some.get returns the value.

If the Option is actually None, then None.get throws a NoSuchElementException.

getOrElse in the last two println statements returns either the value in the Option, if it is a Some instance, or it returns the argument passed to getOrElse, if it is a None instance.

getOrElse argument behaves as the default return value.

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