Case Class In Scala

Case Class is one of the great tool introduced by Scala in the functional programming world.

 case class Book(id: Int, title: String, isbn: Long)

Let’s explore what’s hidden behind the above class.

Case class is similar to a class but it provides a lot of boilerplate code for developers to use. By default, they are immutable i.e. once declared cannot be changed.

If we compile the case class using scalac command and decompile it using javap command, following code is generated:

public class Book implements scala.Product,scala.Serializable {
 public static scala.Option unapply(Book);
 public static Book apply(int, java.lang.String, long);
 public static scala.Function1 tupled();
 public static scala.Function1 curried();
 public int id();
 public java.lang.String title();
 public long isbn();
 public Book copy(int, java.lang.String, long);
 public int copy$default$1();
 public java.lang.String copy$default$2();
 public long copy$default$3();
 public java.lang.String productPrefix();
 public int productArity();
 public java.lang.Object productElement(int);
 public scala.collection.Iterator productIterator();
 public boolean canEqual(java.lang.Object);
 public int hashCode();
 public java.lang.String toString();
 public boolean equals(java.lang.Object);
 public Book(int, java.lang.String, long);
}

Case classes automatically generate the following:

• Getter methods for the constructor arguments.

• Hashcode and equals.

• Copy method.

• Companion object with apply/unapply.

• toString.

If a case class is a data holder, then a corresponding companion object is a service for that case class. Singleton object serves the purpose of both a factory and an extractor.

As a factory, it provides apply method which allows us to create an object without new keyword.

val book = Book(1,"Programming In Scala",9788370017361L)
book: Book = Book(1,Programming In Scala,9788370017361)

As an extractor, it provides unapply method which is used for pattern matching.

book match { case Book(1,"Programming In Scala",9788370017361L) => print("Extractor") case _=> print("Failed!") } Extractor

Case Class constructor parameters are promoted to members. Scala prefixes all the parameters with val. For vals only getters are provided.

scala> book.title
res0: String = Programming In Scala
scala> book.title = "Scala for Impatient"
:12: error: reassignment to val
   book.title = "Scala for Impatient"

We can change Case Class constructor parameters to vars.  For vars getters and setters both are provided.

scala> case class Book(var id: Int, var title: String, var isbn: Long)
 scala> book.title = "Scala for Impatient"
book.title: String = Scala for Impatient

Case classes without parameters are meaningless and deprecated. In that situation, case objects are used.

scala> case class Book
 :1: error: case classes must have a parameter list; try 'case class Book()' or 'case object Book'
scala> case class Book()
defined class Book

Case objects can be created by simply calling the case object with empty parentheses which internally calls apply method and wherever we use the case class it refers to the companion object.

scala> Book() //calls apply method
res3: Book = Book()
scala> Book.apply()
res5: Book = Book()
scala> Book // It references the companion object Book
res6: Book.type = Book

The copy method of case class allows full copy as well as copy using custom values.

scala> val fullCopiedObject = book.copy()
fullCopiedObject: Book = Book(1,Programming In Scala,9788370017361)
scala> val partialCopiedObject = book.copy(title = "Scala for Impatient")
partialCopiedObject: Book = Book(1,Scala for Impatient,9788370017361)

equals method that does the comparison on basis of values.

The == also prints true and it is something called a synthetic method or bridge method which actually calls the equals method.

The eq method compares the memory address of the objects. The ne method is the exact opposite of eq.

scala> book == fullCopiedObject
res12: Boolean = true
scala> book.equals(fullCopiedObject)
res13: Boolean = true
scala> book.eq(fullCopiedObject)
res14: Boolean = false
scala> fullCopiedObejct.ne(book1)
res15: Boolean = true

hashCode method  calls scala.runtime.ScalaRunTime._hashCode which is defined as:

def _hashCode(x: Product): Int = { val arr = x.productArity var code = arr var i = 0 while (i < arr) { val elem = x.productElement(i) code = code * 41 + (if (elem == null) 0 else elem.hashCode()) i += 1 } code }

what you get is elem1 * 41**n + elem2 * 41**(n-1) .. elemn * 1, where n is the arity of case class and elemi are the members of that case class.

Case classes use an algorithm called murmur hash and regular classes use the default hash. MurmurHash was chosen for Scala specifically because of measured defects in the simpler typical Java-style hash, and it has generally worked out well.

toString method in case classes does not give the hexadecimal representation of the hashcode rather it gives a meaningful representation of the class.

scala> book.toString
res16: String = Book(1,Programming In Scala,9788370017361)

In the next blog, we will be discussing case class inheritance. Inheritance in case classes is difficult because of equals and hashcode methods.

Thanks for reading!