If you’ve played around with the Option class, you know that it allows you to note that no value is available and do so without nulls. For example, using getOrElse() you can access a value or a default when no value is present:
scala> val a:Option[Int] = Some(5)
a: Option[Int] = Some(5)
scala> val b:Option[Int] = None
b: Option[Int] = None
scala> a.getOrElse(0)
res1: Int = 5
scala> b.getOrElse(0)
res2: Int = 0
But a problem you’re likely to come across is how to access an instance variable when a class is stored in an option? For example,
scala> case class S(lines:Int, pages:Int)
defined class S
scala> val sOpt:Option[S] = Some(new S(16,5) )
sOpt: Option[S] = Some(S(16,5))
scala> val nOpt:Option[S] = None
nOpt: Option[S] = None
Here, we’ve created a simple class an put an instance of it into an Option. Then we made another option container for the class containing None. So given an Option[S], how do you get the lines value out of it? You can’t do it directly because the Option may contain None.
The answer is to use map, passing in the accessor or other function you want to apply. In this case, we just use the accessor lines. An Option is returned, so getOrElse() then returns the value or a default.
scala> val numLines = sOpt.map{ _.lines }.getOrElse(0)
numLines: Int = 16
scala> val numLines = nOpt.map{ _.lines }.getOrElse(0)
numLines: Int = 0
The underscore is an abbreviation for the object in the map. Don’t forget that you can also pass functions into the map.
scala> sOpt.map{println}
S(16,5)
scala> nOpt.map{println}
Not that the instance toString() was printed by the first line, whereas the second printed nothing because the Option contained a None. (I’ve omitted the REPL result lines.)
Finally, note that complex functions can be built using the normal block “=>” syntax. Here, we print as well as accessing the lines instance variable:
scala> val numLines = sOpt.map{x => println(x); x.lines}.getOrElse(0)
S(16,5)
numLines: Int = 16
scala> val numLines = nOpt.map{x => println(x); x.lines}.getOrElse(0)
numLines: Int = 0
scala>
