Should I avoid defining 'object' in Scala? - scala

Question

I think 'object' in Scala is pretty similar to Singleton in Java which is not considered to be a good design practice. Singleton to me is like another way to define global variables which is BAD. I wrote some Scala code like this because it's easy and it works but the code looks ugly:
object HttpServer { // I'm the only HttpServer instance in this program.
var someGlobalState: State
def run() {
// do something
}
}
I'm trying to avoid doing this. When is it good to define Scala object?

Answer 1

No. Many Scala-Libraries heavily rely on object.
The main goal of the Singleton-Pattern is that just one instance of the Object can exist. The same holds true for Object.
You may misuse it as global variable but that is not the point.
Object are for example a great place for Factory Methods or a replacement for Modules to hold functions.

Answer 2

Why do you assume that you only want global variables? Global values and methods are really useful. This is most of what you'll use object for in Scala.
object NumericConstant {
val Pi = 3.1415926535897932385 // I probably will not change....
}
object NumericFunctions {
def squared(x: Double) = x*x // This is probably always what we mean...
}
Now, you do have to be careful using global variables, and if you want to you can implement them in objects. Then you need to figure out whether you are being careless (note: passing the same instance of a class to every single class and method in your program is equally problematic), or whether the logic of what you are doing really is best reflected by a single global value.
Here's a really, really bad idea:
object UserCache {
var newPasswordField: String = "foo bar"
}
Two users change their password simultaneously and...well...you will have some unhappy users.
On the other hand,
object UserIDProvider {
private[this] var maxID = 1
def getNewID() = this.synchronized {
var id = maxID
maxID += 1
id
}
}
if you don't do something like this, again, you're going to have some unhappy users. (Of course, you'd really need to read some state on disk regarding user ID number on startup...or keep all that stuff in a database...but you get the point.)

Answer 3

Global variables are not inherently bad. You just need to understand when it's appropriate. And so it follows that object is not inherently bad. For example:
object HelloWorld {
def main(args:Array[String]){
println("Hello World")
}
}
Without going into a long discussion of the topic, I like to think of it this way: "Do I want 'only one' of these things because that best reflects reality? Or is this a lazy shortcut to get things to 'just work'?"
Don't just blindly and broadly apply the "Singleton is bad" rule. There are plenty of cases where "just one" of something makes sense. In your particular case, I'd need more context to give a more specific recommendation.