About nulls and System Properties in Scala

In Scala, it is usually a good idea to avoid nulls in your code. Instead of that, it is recommended that you use the Option class. This is how it works: if the value you are working with is null, it is represented by None, otherwise, it is represented by Some(value). Both None and Some are subclasses of Option.

This allows you to write code that is much more expressive than having ‘if != null‘ all over the place. Also, it is much less likely that you will forget to verify for nullity. One way of leveraging an Option value is through pattern matching:

val myValue: Option[String] = Some("Hello")
myValue match {
  case Some(x) => println("my value is " + x)
  case None => println("nonthing found")
}

The Option class also offers some collection-like functions that allow for great readability as well. From the Scaladoc:

val name: Option[String] = request.getParameter("name")
val upper = name map { _.trim } filter { _.length != 0 } map { _.toUpperCase }
println(upper.getOrElse(""))

Notice how the code execute transformations and ‘if-like’ filtering, in a very clean way. After that, they try to use the value with the getOrElse function, which allows you to pass a default value to be used if the result from the previous line was None. Finally, if you REALLY want to use an if statement, you can call the isDefined function, which returns true if the value is Some, false otherwise.

Now lets talk about system properties. Lets face it: dealing with System Properties in Java is boring. You have to get each property you are interested in and have to verify if it is null or empty. After all the talk above, you probably are starting to guess where I’m heading, right?

Well, it is better than that. Scala encapsulates the system properties in a way we can leverage all the Option stuff explained above. Those properties are encapsulated in a class named SystemProperties and can be accessed by the object ‘sys.props‘. Try typing it into a scala console session to see for yourself.

Since it is a map, you can use any map function you can use with other maps. Also, you can add and get properties from this object, and they will be reflected in the actual System Properties – which means that your Java code should be able to read the properties as well, even if through more traditional means (i.e. System.getProperty). Printing all system properties is as simple as:

sys.props.foreach(println)

And you clearly could do something more clever and useful with those properties.

There are two functions available in the SystemProperties class that I want to cover here: get and getOrElse. The first one returns a property with a given name, in an Option. With this, you can now handle your property with all the Option goodness we talked about above. The second function, getOrElse, is basically a direct way of calling the same function you could call in an Option. For example, lets say you read a database configuration from the system properties, and want to fall back to a default configuration if the property is not found:

val dbURL = sys.props.getOrElse("myapp.database.url", "jdbc:postgresql://localhost:5432/mydb")

A lot better than reading the property and verifying if it exists ‘by hand‘, right?

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One Response to About nulls and System Properties in Scala

  1. Pingback: Sobre nulls e propriedades do sistema em Scala | iMasters

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