Scala Book

Supplying Default Values for Constructor Parameters

Language

Scala lets you supply default values for constructor parameters. For example, in previous lessons we 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 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)

Here’s 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

Benefits

Supplying default constructor parameters has at least two benefits:

  • You provide preferred, default values for your parameters
  • You let consumers of your class override those values for their own needs

As shown in the examples, a third benefit is that it lets consumers construct new Socket instances in at least three different ways, as if it had three class constructors.

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)

This feature comes in handy from time to time, such as when all of the class constructor parameters have the same type, such as the Int parameters 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)

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