Companion Object

A companion object is an object with the same name as a class or trait and is defined in the same source file as the associated file or trait. A companion object differs from other objects as it has access rights to the class/trait that other objects do not. In particular it can access methods and fields that are private in the class/trait.

An analog to a companion object in Java is having a class with static methods. In Scala you would move the static methods to a Companion object.

One of the most common uses of a companion object is to define factory methods for class. An example is case-classes. When a case-class is declared a companion object is created for the case-class with a factory method that has the same signature as the primary constructor of the case class. That is why one can create a case-class like: MyCaseClass(param1, param2). No new element is required for case-class instantiation.

A second common use-case for companion objects is to create extractors for the class. I will mention extractors in a future topic. Basically extractors allow matching to work with arbitrary classes.

NOTE: Because the companion object and the class must be defined in the same source file you cannot create them in the interpreter. So copy the following example into a file and run it in script mode:

scala mysourcefile.scala


Example:

1. class MyString(val jString:String) {
2.   private var extraData = ""
3.   override def toString = jString+extraData
4. }
6. object MyString {
7.   def apply(base:String, extras:String) = {
8.     val s = new MyString(base)
9.     s.extraData = extras
10.     s
11.   }
12.   def apply(base:String) = new MyString(base)
13. }
15. println(MyString("hello"," world"))
16. println(MyString("hello"))