Why does Scala have a case object?

Question

It cannot be initialized with params and it will only be compared with itself (singleton). So no advantage for its equals and hash code function. Has anyone come across a case where they find it useful?

Answer 1

You can use case objects as an alternative for enumerations.

Details can be found here: Case objects vs Enumerations in Scala

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A simplified example from that question:

sealed trait Currency {
  def name: String
  def symbol: String
}

case object EUR extends Currency {
  val name = "EUR"
  val symbol = "€"
}

case object USD extends Currency {
  val name = "USD"
  val symbol = "$"
}

Advantages

• This way there can be more fields (compared to ID and name in an Enumeration)
• The compiler warns (in case of a sealed type hierarchy), if a match is not exhaustive.

So this code

val ccy: Currency = EUR
ccy match {
  case EUR =>
    println("Euro")
}

will result in

Warning:(27, 7) match may not be exhaustive.
It would fail on the following inputs: USD
ccy match {

Disadvantages

• There is no "get by name" method (Enumeration provides withName())
• You cannot iterate over "all" elements

Answer 2

I think the most important difference is that case objects can be serialized while simple objects cannot.

This makes them very useful as messages with Akka-Remote.

EDIT: As Rüdiger Klaehn pointed out, this is not the only benefit we get from the case keyword. There is also:

• hashCode implementation
• a useful toString implementation

For classes additionally:

• pattern matching optimization
• a companion object with useful apply and unapply implementations

(This list may not be exhaustive!)