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Scala trait mixins allow you to extend any number of traits with a class or abstract class. Scala traits, or a combination of traits and classes, or traits and abstract classes, can be extended using trait mixins.

Trait Mixins - Extending Traits and Mixing Them In

Scala traits provide a powerful way to compose behavior from multiple sources. You can mix in multiple traits into a single class to combine their behavior. Let's extend the previous example to include additional traits and see how they interact.

Example

trait Printable extends Any {
   def print(): Unit = println(this)
}

trait Movable {
   def move(dx: Int, dy: Int): Unit
}

trait Equal {
   def isEqual(obj: Any): Boolean
   def isNotEqual(obj: Any): Boolean = !isEqual(obj)
}

class Point(xc: Int, yc: Int) extends Equal with Printable with Movable {
   var x: Int = xc
   var y: Int = yc

   def isEqual(obj: Any): Boolean = obj.isInstanceOf[Point] 
   && obj.asInstanceOf[Point].x == x && obj.asInstanceOf[Point].y == y

   def move(dx: Int, dy: Int): Unit = {
      x += dx
      y += dy
   }

   override def print(): Unit = println(s"Point($x, $y)")
}

object Demo {
   def main(args: Array[String]): Unit = {
      val p1 = new Point(2, 3)
      val p2 = new Point(2, 4)
      val p3 = new Point(2, 3)

      println(p1.isNotEqual(p2))
      println(p1.isNotEqual(p3))
      println(p1.isNotEqual(2))

      p1.print()
      p1.move(1, 1)
      p1.print()
   }
}

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

Command

\>scalac Demo.scala
\>scala Demo

The output will be −

true
false
true
Point(2, 3)
Point(3, 4) 

Here, the Point class now mixes in the Printable and Movable traits in addition to the Equal trait. You can have multiple traits combined for rich set of behaviors to a class. The Point class implements the move method from the Movable trait. It overrides the print method from the Printable trait to provide a custom string representation.

Advanced Trait Mixins

You can also define fields in traits for complex behaviors. Following is the example which shows you how advanced trait mixins −

Example

trait Equal {
   def isEqual(obj: Any): Boolean
}

trait Printable {
   def print(): Unit
}

trait Movable {
   def move(dx: Int, dy: Int): Unit
}

trait Timestamped {
   var timestamp: Long = System.currentTimeMillis()

   def updateTimestamp(): Unit = {
      timestamp = System.currentTimeMillis()
   }
}

class Point(xc: Int, yc: Int) extends Equal with Printable with Movable with Timestamped {
   var x: Int = xc
   var y: Int = yc

   def isEqual(obj: Any): Boolean = obj.isInstanceOf[Point] && obj.asInstanceOf[Point].x == x && obj.asInstanceOf[Point].y == y

   def move(dx: Int, dy: Int): Unit = {
      x += dx
      y += dy
      updateTimestamp()
   }

   override def print(): Unit = println(s"Point($x, $y) at $timestamp")
}

object Demo {
   def main(args: Array[String]): Unit = {
      val p1 = new Point(2, 3)
      p1.print()
      p1.move(1, 1)
      p1.print()
   }
}

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

Command

\>scalac Demo.scala
\>scala Demo

The output will be −

Point(2, 3) at 1628609214000
Point(3, 4) at 1628609215000

Here, the Point class also mixes in the Timestamped trait. It has field and method to update the timestamp. So you can use traits to add fields and methods to a class to enrich its behavior and state.

Trait Mixins Summary

• Traits are used to compose behavior in Scala. You can mix multiple traits into a single class to combine their functionalities.
• You can use traits to define reusable behaviors that can be mixed into multiple and unrelated classes. So, it promotes code reuse and modularity.
• You can have partially implemented methods in traits. So you can provide default behavior that can be overridden by classes mixing in the trait.
• You can also define fields, which can be used for adding state to classes that mix in the trait.
• You can use universal traits for value classes to extend traits.