Using Option with .map() and .getOrElse() - scala

Question

I am trying to read a value from a Map[String, String] given a key.
This key|value is optional, in that it might not be there
So, I want to use Option and then map & getOrElse as below to write the value if it's there, or set it to some default in case it's not there.
val endpoint:String = Option(config.getString("endpoint"))
.map(_.value())
.getOrElse()
The code above fails with "Symbol value is inaccessible from this place"
config is a Map[String, Object]
getString is a method on config that takes in the key, and returns the value
public String getString(String key){
<...returns value...>
}
I could just drop the Option() and do, but then I have to deal with the exception that will be throw by getString()
val endpoint:String = config.getString("endpoint")
Any ideas what's wrong with this, or how to fix this?
Better ways of writing this?
UPDATE: I need to mention that config is an object in an imported Java library. Not sure if that makes a difference or not.

Answer 1

If I understand your question correctly, config.getString will throw an exception when the key is not present. In this case, wrapping the call in Option() will not help catch that exception: you should wrap in Try instead and convert that to an Option.
Try[String] represents a computation that can either succeed and become a Success(String), or fail and give you a Failure(thrownException). If you're familiar with Option, this is very similar to the two possibilities of Some and None, except that Failure will wrap the exception so that you know what caused the problem. The Try(someComputation) method will just do something like this for you:
try {
Success(someComputation)
} catch {
case ex: Exception => Failure(ex)
}
The second thing to consider is what you actually want to happen when there is no value. One sensible idea would be to provide a default configuration, and this is what getOrElse is for: you can't use without giving it the default value!
Here is an example:
val endpoint = Try(config.getString("endpoint"))
.toOption
.getOrElse("your_default_value")
We can do even better: now that we're using Try to catch the exception, there is no need to convert to Option if we're going to access the value right away.
val endpoint = Try(config.getString("endpoint")).getOrElse("your_default_value")

Answer 2

You can get a value from a map like this.
val m: Map[String, String] = Map("foo" -> "bar")
val res = m.get("foo").getOrElse("N.A")
val res2 = m.getOrElse("foo", "N.A") // same as above but cleaner
But perhaps if you want to use pattern matching:
val o: Option[String] = m.get("foo")
val res: String = o match {
case Some(value) => value
case None => "N.A"
}
Finally, a safe way to handle reading from config.
val endpoint:String = config.getString("endpoint") // this can return null
val endpoint: Option[String] = Option(config.getString("endpoint")) // this will return None if endpoint is not found
I suspect the config object might even have a method like
val endpoint: Option[String] = config.getStringOpt("endpoint")
Then you can use pattern matching to extract the value in the option. Or one of the many combinators map, flatMap, fold etc
val endPoint = Option(config.getString("endpoint"))
def callEndPoint(endPoint: String): Future[Result] = ??? // calls endpoint
endPoint match {
case Some(ep) => callEndPoint(ep)
case None => Future.failed(new NoSuchElementException("End point not found"))
}
Or
val foo = endPoint.map(callEndPoint).getOrElse(Future.failed(new NoSuchElement...))