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Scala Sealed Trait

By IncludeHelp Last updated : October 26, 2024

Sealed Trait

Sealed Traits in Scala are used to allow exhaustive checking in the Scala application. Due to this exhaustive checking, the program allows the members of the sealed trait to be declared in a single file.

This helps the compiler by providing a list of all possible members of the trait beforehand. Which reduces the chances of errors in our code.

Syntax

Let's see the block of code, here we have a trait whose all members are declared with it.

sealed trait trait1{}
class class1 extends trait1
class class2 extends trait1
class class3 extends trait1

Now since we have a sealed trait, exhaustive checking will we used which will require all the sub-types of the trait to be included otherwise it will throw a warning.

Example of Sealed Trait

sealed trait RoyalEnfield { 
  val speed: Int 
} 

class Bullet extends RoyalEnfield { 
  override val speed: Int = 125 
} 

class Thunderbird extends RoyalEnfield { 
  override val speed: Int = 137 
} 

class Continental extends RoyalEnfield { 
  override val speed: Int = 152 
} 

object MyObject { 
  def main(args: Array[String]): Unit = { 
    val bullet350 = new Bullet 
    val thunderbird350 = new Thunderbird 
    val continentalGT = new Continental 
    println(topSpeed(bullet350)) 
    println(topSpeed(thunderbird350)) 
    println(topSpeed(continentalGT)) 
  } 

  def topSpeed(article: RoyalEnfield): Int = article match { 
    case bullet: Bullet => bullet.speed 
    case thunderbird: Thunderbird => thunderbird.speed 
    case continental: Continental => continental.speed 
  } 
}

Output

125
137
152

Features of Sealed Trait

Key features of sealed traits in Scala include the following:

• The compiler knows all the sub-types of the trait in advance which help it avoiding errors by throwing warnings.
• The inheritance of a sealed trait is allowed only by classes inside the file that declares the trait. In an external file, if a class tries to extend the trait error will be thrown by the compiler.
• To prevent the above type of inheritance, sealed traits are generally used with enums.