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Scala | unapplySeq() method

  • Last Updated : 08 Apr, 2019

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 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 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.




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