Case classes are like regular classes with a few key differences which we will go over. Case classes are good for modeling immutable data. In the next step of the tour, we’ll see how they are useful in pattern matching.
Defining a case class
A minimal case class requires the keywords
case class, an identifier, and a parameter list (which may be empty):
case class Book(isbn: String) val frankenstein = Book("978-0486282114")
Notice how the keyword
new was not used to instantiate the
Book case class. This is because case classes have an
apply method by default which takes care of object construction.
When you create a case class with parameters, the parameters are public
case class Message(sender: String, recipient: String, body: String) val message1 = Message("email@example.com", "firstname.lastname@example.org", "Ça va ?") println(message1.sender) // prints email@example.com message1.sender = "firstname.lastname@example.org" // this line does not compile
You can’t reassign
message1.sender because it is a
val (i.e. immutable). It is possible to use
vars in case classes but this is discouraged.
Case classes are compared by structure and not by reference:
case class Message(sender: String, recipient: String, body: String) val message2 = Message("email@example.com", "firstname.lastname@example.org", "Com va?") val message3 = Message("email@example.com", "firstname.lastname@example.org", "Com va?") val messagesAreTheSame = message2 == message3 // true
message3 refer to different objects, the value of each object is equal.
You can create a (shallow) copy of an instance of a case class simply by using the
copy method. You can optionally change the constructor arguments.
case class Message(sender: String, recipient: String, body: String) val message4 = Message("email@example.com", "firstname.lastname@example.org", "Me zo o komz gant ma amezeg") val message5 = message4.copy(sender = message4.recipient, recipient = "email@example.com") message5.sender // firstname.lastname@example.org message5.recipient // email@example.com message5.body // "Me zo o komz gant ma amezeg"
The recipient of
message4 is used as the sender of
message5 but the
message4 was copied directly.