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scalasetcompanion-object

Companion object for Set in Scala

From book 'Programming in Scala',doing this:

var jetSet = Set("Boeing", "Airbus")

invokes 'apply' on the companion object for scala.collection.immutable.Set, which returns an instance of a default, immutable Set.

But isn't companion object defined for the class with same name, and scala.collection.immutable.Set is a trait and not a class.So how come we have companion object for a trait?

Solution

  • Scala allows you to define a companion object for both traits and classes.

    Try it:

    trait Foo

object Foo {
    def apply(x: Int): Foo = new Foo { }
}

    Ih this example the companion object returns an instance of type Foo that is implemented as an anonymous class that extends the trait.