Tour of Scala

Package Objects

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Package objects

Scala provides package objects as a convenient container shared across an entire package.

Package objects can contain arbitrary definitions, not just variable and method definitions. For instance, they are frequently used to hold package-wide type aliases and implicit conversions. Package objects can even inherit Scala classes and traits.

By convention, the source code for a package object is usually put in a source file named package.scala.

Each package is allowed to have one package object. Any definitions placed in a package object are considered members of the package itself.

See example below. Assume first a class Fruit and three Fruit objects in a package gardening.fruits:

// in file gardening/fruits/Fruit.scala
package gardening.fruits

case class Fruit(name: String, color: String)
object Apple extends Fruit("Apple", "green")
object Plum extends Fruit("Plum", "blue")
object Banana extends Fruit("Banana", "yellow")

Now assume you want to place a variable planted and a method showFruit directly into package gardening. Here’s how this is done:

// in file gardening/fruits/package.scala
package gardening
package object fruits {
  val planted = List(Apple, Plum, Banana)
  def showFruit(fruit: Fruit): Unit = {
    println(s"${fruit.name}s are ${fruit.color}")
  }
}

As an example of how the use site looks, the following object PrintPlanted imports planted and showFruit in exactly the same way it imports class Fruit, using a wildcard import on package gardening.fruits:

// in file PrintPlanted.scala
import gardening.fruits._
object PrintPlanted {
  def main(args: Array[String]): Unit = {
    for (fruit <- fruits.planted) {
      showFruit(fruit)
    }
  }
}

Package objects are like other objects, which means you can use inheritance for building them. For example, one might mix in a couple of traits:

package object fruits extends FruitAliases with FruitHelpers {
  // helpers and variables follows here
}

Note that method overloading doesn’t work in package objects.

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