Scala | unapplySeq() method

The unapplySeq() method is an Extractor method. It extracts an Object of particular type and then again reconstructs it into a Sequence of extracted values and the length of this Sequence is not specified at the time of compilation. So, in order to reconstruct an Object that contains a Sequence, you need to utilize this unapplySeq method. Syntax:

def unapplySeq(object: X): Option[Seq[T]]

Here, we have an object of type X and this method either returns None, when the object does not match or returns a Sequence of extracted values of type T, enclosed in class Some. Now, lets understand it through some examples. Example :

Scala

// Scala program of unapplySeq
// method

// Creating object
object GfG
{

    // Defining unapplySeq method
    def unapplySeq(x:Any): Option[Product2[Int,Seq[String]]] = 
    {

        val y = x.asInstanceOf[Author] 
        
        if(x.isInstanceOf[Author]) 
        {
            Some(y.age, y.name)
        }     
        else
            None
    } 

    // Main method
    def main(args:Array[String]) = 
    {

        // Creating object for Author
        val x = new Author

        // Applying Pattern matching
        x match
        {
            case GfG(y:Int,_,z:String) =>

            // Displays output
            println("The age of "+z+" is: "+y)
        }

        // Assigning age and name
        x.age = 22
        x.name = List("Rahul","Nisha")

        // Again applying Pattern matching
        x match
        {                             
            case GfG(y:Int,_,z:String) => 
    
            //Displays output
            println("The age of "+z+" is: "+y)
        }
    }
}

// Creating class for author
class Author
{

    // Assigning age and name
    var age: Int = 24
    var name: Seq[String] = List("Rohit","Nidhi")
}

Output:

The age of Nidhi is: 24
The age of Nisha is: 22

Here, we have used a trait Product2 in the Option in order to pass two arguments to it. Product2 is a Cartesian product of two elements. Example :

Scala

// Scala program of using 
//'UnapplySeq' method of 
// Extractors

// Creating object
object GfG 
{

    // Main method
    def main(args: Array[String]) 
    {
        object SortedSeq
        {
    
            // Defining unapply method
            def unapplySeq(x: Seq[Int]) = 
            {
                if (x == x.sortWith(_ < _)) 
    
                {
                    Some(x) 
                }
                else None
            }
        }
        
        // Creating a List                     
        val x = List(1,2,3,4,5) 

        // Applying pattern matching
        x match 
        { 
            case SortedSeq(a, b, c, d,e) =>

        // Displays output
        println(List(a, c, e))
        } 
    }
}

Output:

List(1, 3, 5)

Here, we have used a function sortWith, that sorts the stated sequence as specified by the comparison function.

Scala | unapplySeq() method

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