Scala Cookbook

6.6. Creating Static Members with Companion Objects

Problem

You want to create a class that has instance methods and static methods, but unlike Java, Scala does not have a static keyword.

Solution

Define nonstatic (instance) members in your class, and define members that you want to appear as “static” members in an object that has the same name as the class, and is in the same file as the class. This object is known as a companion object.

Using this approach lets you create what appear to be static members on a class (both fields and methods), as shown in this example:

// Pizza class
class Pizza (var crustType: String) {
  override def toString = "Crust type is " + crustType
}

// companion object
object Pizza {
  val CRUST_TYPE_THIN = "thin"
  val CRUST_TYPE_THICK = "thick"
  def getFoo = "Foo"
}

With the Pizza class and Pizza object defined in the same file (presumably named Pizza.scala), members of the Pizza object can be accessed just as static members of a Java class:

println(Pizza.CRUST_TYPE_THIN)
println(Pizza.getFoo)

You can also create a new Pizza instance and use it as usual:

var p = new Pizza(Pizza.CRUST_TYPE_THICK)
println(p)

Note

If you’re coming to Scala from a language other than Java, “static” methods in Java are methods that can be called directly on a class, without requiring an instance of the class. For instance, here’s an example of a method named increment in a Scala object named StringUtils:

object StringUtils {
  def increment(s: String) = s.map(c => (c + 1).toChar)
}

Because it’s ...