Scala Book

Case Classes


Another Scala feature that provides support for functional programming is the case class. A case class has all of the functionality of a regular class, and more. When the compiler sees the case keyword in front of a class, it generates code for you, with the following benefits:

  • Case class constructor parameters are public val fields by default, so accessor methods are generated for each parameter.
  • An apply method is created in the companion object of the class, so you don’t need to use the new keyword to create a new instance of the class.
  • An unapply method is generated, which lets you use case classes in more ways in match expressions.
  • A copy method is generated in the class. You may not use this feature in Scala/OOP code, but it’s used all the time in Scala/FP.
  • equals and hashCode methods are generated, which let you compare objects and easily use them as keys in maps.
  • A default toString method is generated, which is helpful for debugging.

These features are all demonstrated in the following sections.

With apply you don’t need new

When you define a class as a case class, you don’t have to use the new keyword to create a new instance:

scala> case class Person(name: String, relation: String)
defined class Person

// "new" not needed before Person
scala> val christina = Person("Christina", "niece")
christina: Person = Person(Christina,niece)

As discussed in the previous lesson, this works because a method named apply is generated inside Person’s companion object.

No mutator methods

Case class constructor parameters are val fields by default, so an accessor method is generated for each parameter:

res0: String = Christina

But, mutator methods are not generated:

// can't mutate the `name` field
scala> = "Fred"
<console>:10: error: reassignment to val = "Fred"

Because in FP you never mutate data structures, it makes sense that constructor fields default to val.

An unapply method

In the previous lesson on companion objects you saw how to write unapply methods. A great thing about a case class is that it automatically generates an unapply method for your class, so you don’t have to write one.

To demonstrate this, imagine that you have this trait:

trait Person {
    def name: String

Then, create these case classes to extend that trait:

case class Student(name: String, year: Int) extends Person
case class Teacher(name: String, specialty: String) extends Person

Because those are defined as case classes — and they have built-in unapply methods — you can write a match expression like this:

def getPrintableString(p: Person): String = p match {
    case Student(name, year) =>
        s"$name is a student in Year $year."
    case Teacher(name, whatTheyTeach) =>
        s"$name teaches $whatTheyTeach."

Notice these two patterns in the case statements:

case Student(name, year) =>
case Teacher(name, whatTheyTeach) =>

Those patterns work because Student and Teacher are defined as case classes that have unapply methods whose type signature conforms to a certain standard. Technically, the specific type of pattern matching shown in these examples is known as a constructor pattern.

The Scala standard is that an unapply method returns the case class constructor fields in a tuple that’s wrapped in an Option. The “tuple” part of the solution was shown in the previous lesson.

To show how that code works, create an instance of Student and Teacher:

val s = Student("Al", 1)
val t = Teacher("Bob Donnan", "Mathematics")

Next, this is what the output looks like in the REPL when you call getPrintableString with those two instances:

scala> getPrintableString(s)
res0: String = Al is a student in Year 1.

scala> getPrintableString(t)
res1: String = Bob Donnan teaches Mathematics.

All of this content on unapply methods and extractors is a little advanced for an introductory book like this, but because case classes are an important FP topic, it seems better to cover them, rather than skipping over them.

copy method

A case class also has an automatically-generated copy method that’s extremely helpful when you need to perform the process of a) cloning an object and b) updating one or more of the fields during the cloning process. As an example, this is what the process looks like in the REPL:

scala> case class BaseballTeam(name: String, lastWorldSeriesWin: Int)
defined class BaseballTeam

scala> val cubs1908 = BaseballTeam("Chicago Cubs", 1908)
cubs1908: BaseballTeam = BaseballTeam(Chicago Cubs,1908)

scala> val cubs2016 = cubs1908.copy(lastWorldSeriesWin = 2016)
cubs2016: BaseballTeam = BaseballTeam(Chicago Cubs,2016)

As shown, when you use the copy method, all you have to do is supply the names of the fields you want to modify during the cloning process.

Because you never mutate data structures in FP, this is how you create a new instance of a class from an existing instance. This process can be referred to as, “update as you copy.”

equals and hashCode methods

Case classes also have automatically-generated equals and hashCode methods, so instances can be compared:

scala> case class Person(name: String, relation: String)
defined class Person

scala> val christina = Person("Christina", "niece")
christina: Person = Person(Christina,niece)

scala> val hannah = Person("Hannah", "niece")
hannah: Person = Person(Hannah,niece)

scala> christina == hannah
res1: Boolean = false

These methods also let you easily use your objects in collections like sets and maps.

toString methods

Finally, case classes also have a good default toString method implementation, which at the very least is helpful when debugging code:

scala> christina
res0: Person = Person(Christina,niece)

The biggest advantage

While all of these features are great benefits to functional programming, as they write in the book, Programming in Scala (Odersky, Spoon, and Venners), “the biggest advantage of case classes is that they support pattern matching.” Pattern matching is a major feature of FP languages, and Scala’s case classes provide a simple way to implement pattern matching in match expressions and other areas.

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