Supplying Default Values for Constructor Parameters

A convenient Scala feature is that you can supply default values for constructor parameters. In the previous lessons I showed that you can define a Socket class like this:

class Socket(var timeout: Int, var linger: Int) {
    override def toString = s"timeout: $timeout, linger: $linger"
}

That’s nice, but you can make this class even better by supplying default values for the timeout and linger parameters:

class Socket(var timeout: Int = 2000, var linger: Int = 3000) {
    override def toString = s"timeout: $timeout, linger: $linger"
}

By supplying default values for the parameters, you can now create a new Socket in a variety of different ways:

new Socket()
new Socket(1000)
new Socket(4000, 6000)

This is what those examples look like in the REPL:

scala> new Socket()
res0: Socket = timeout: 2000, linger: 3000

scala> new Socket(1000)
res1: Socket = timeout: 1000, linger: 3000

scala> new Socket(4000, 6000)
res2: Socket = timeout: 4000, linger: 6000

Bonus: Named parameters

Another nice thing about Scala is that you can used named parameters when creating a new instance of a class. For instance, given this class:

class Socket(var timeout: Int, var linger: Int) {
    override def toString = s"timeout: $timeout, linger: $linger"
}

you can create a new Socket like this:

val s = new Socket(timeout=2000, linger=3000)

I rarely use this feature, but it comes in handy every once in a while, especially when all of the class constructor parameters have the same type, such as Int in this example. For example, some people find that this code:

val s = new Socket(timeout=2000, linger=3000)

is more readable than this code:

val s = new Socket(2000, 3000)