Scala | Upper bound

Last Updated : 14 Mar, 2019

Scala has some restrictions on the Type Parameters or Type Variable i.e, (type bound) and Upper bound is one of them. By implementing Type Bounds, We can explicate the restraints of a Type Variable, these type bound restricts the definite values of the type variables and discloses more details about the member of these types. Upper bound is defined on type parameters.
Syntax

[T <: S]

Where, ‘T’ is a type parameter and ‘S’ is a type“[T <: S]” means this Type Parameter T must be either same as S or Sub-Type of S.

Let’s understand upper bound with example.
Example

// Scala Program To Demonstrate Scala Upper Bound
class GeeksforGeeks
class Author extends GeeksforGeeks
class Geeks extends Author
  
class ComputerSciencePortal
{
    // Declaration of upper bound
    def display [T <: Author](d : T)
    {
        println(d)
    }
}
  
// Object created
object ScalaUpperBounds
{
    // Driver code
    def main(args: Array[String])
    {
  
        val geeksforgeeks = new GeeksforGeeks
        val author = new Author
        val geeks = new Geeks
  
        val computerscienceportal = new ComputerSciencePortal
  
    // computerscienceportal.display(geeksforgeeks)
    computerscienceportal.display(geeks)
    computerscienceportal.display(author)
    }
}

Output:

Geeks@506e1b77
Author@4fca772d

Here, Upper bound is defined in class ComputerSciencePortal, and GeeksforGeeks is the Super class of Author so its not accepted.

Example

// Scala Program To Demonstrate Scala Upper Bound
class  Principal
class Teacher extends Principal
class Student extends Teacher
  
class School
{
    // Declaration of upper bound
    def display [T <: Teacher](t: T)
    {
        println(t)
    }
}
  
// Object created
object ScalaUpperBounds
{
    // Driver code
    def main(args: Array[String])
    {
        // Defined new variable
        val principal = new Principal
        val teacher = new Teacher
        val student = new Student
  
        val school = new School
          
        // school.display(principal)
        school.display(teacher)
        school.display(student)
  }
}

Output:

Teacher@506e1b77
Student@4fca772d

Here, we have defined Upper bound in Class School i.e, [T <: Teacher] which implies that display method accepts Teacher class object or subclass of it (i.e. Student) which means that super class of Teacher will not be accepted. So, if Super class of Teacher i.e, Principal is not commented then it will show type mismatch error.

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Scala | Upper bound

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