An extractor in Scala is an object that has a method called unapply as one of its members. The aim of that unapply method is to match a value and take it apart. Often, the extractor object also defines a dual method apply for building values, but this is not required.

Example

Let us take an example of object defines both apply and unapply methods. The apply method has the same meaning as always: it turns Test into an object that can be applied to arguments in parentheses in the equivalent way a method is applied. So you can write Test (“Zara”, “gmail.com”) to construct the string “Zara@gmail.com”.

The unapply method is what turns Test class into an extractor and it reverses the construction process of apply. Where apply takes two strings and forms an email address string out of them, unapply takes an email address and returns potentially two strings: the user and the domain of the address.

The unapply must also handle the case where the given string is not an email address. That’s why unapply returns an Option-type over pairs of strings. Its result is either Some (user, domain) if the string str is an email address with the given user and domain parts, or None, if str is not an email address. Here are some examples as follows.

Syntax

unapply("Zara@gmail.com") equals Some("Zara", "gmail.com")

unapply("Zara Ali") equals None

Subsequent example program shows an extractor object for email addresses.

Example

object Demo {

   def main(args: Array[String]) {

      println ("Apply method : " + apply("Zara", "gmail.com"));

      println ("Unapply method : " + unapply("Zara@gmail.com"));

      println ("Unapply method : " + unapply("Zara Ali"));

   }

  

   // The injection method (optional)

   def apply(user: String, domain: String) = {

      user +"@"+ domain

   }




   // The extraction method (mandatory)

   def unapply(str: String): Option[(String, String)] = {

      val parts = str split "@"

     

      if (parts.length == 2){

         Some(parts(0), parts(1))

      } else {

         None

      }

   }

}

Save the above program in Demo.scala. The subsequent commands are used to compile and execute this program.

Command

\>scalac Demo.scala

\>scala Demo

Output

Apply method : Zara@gmail.com

Unapply method : Some((Zara,gmail.com))

Unapply method : None

Pattern Matching with Extractors

When an instance of a class is followed by parentheses with a list of zero or more parameters, the compiler invokes the apply method on that instance. We can define apply both in objects and in classes.

As mentioned above, the aim of the unapply method is to extract a selected value we are trying to find . It does the opposite operation apply does. When comparing an extractor object using the match statement the unapply method will be automatically executed.

Try the subsequent example program.

Example

object Demo {

   def main(args: Array[String]) {

      val x = Demo(5)

      println(x)




      x match {

         case Demo(num) => println(x+" is greater twice times than "+num)

        

         //unapply is invoked

         case _ => println("i cannot calculate")

      }

   }

   def apply(x: Int) = x*2

   def unapply(z: Int): Option[Int] = if (z%2==0) Some(z/2) else None

}

Save the above program in Demo.scala. The subsequent commands are used to compile and execute this program.

Command

\>scalac Demo.scala

\>scala Demo

Output

10 is greater twice times than 5

So, this brings us to the end of blog. This Tecklearn ‘Extractor Object in Scala and how to perform pattern matching using extractors’ helps you with commonly asked questions if you are looking out for a job in Apache Spark and Scala and Big Data Developer. If you wish to learn Apache Spark and Scala and build a career in Big Data Hadoop domain, then check out our interactive, Apache Spark and Scala Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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