Scala Wonderland: Semicolons, singletons and companion objects

In Scala you may usually omit semicolon at the end of a statement. It is required if more statements are on a single line. Unfortunately there are cases when compiler doesn’t undrstand the code as you would expect. For example following is treated as two statements a and +b:

a + b

Solution is to use parenthesis (a + b).

Scala doesn’t have static members. It has singleton objects instead. Syntactically it looks like a class, except you use the keyword object. The main benefit is that an object can extend a class or mix in traits. On the other hand thay cannot take (constructor) parameters.

class Planet
object Earth extends Planet
object Sun extends Planet

When a singleton has the same name as a class, it’s called to be a companion object of that class. The class is denoted as the companion class. They both must be implemented in the same file. The beauty is that they can access each other’s private members. Typical usage is to have factory and helper methods in the companion object.

class Sheep(val name: String) {
  private var isBlack = false
}

object Sheep {
  // Factory method #1
  def apply(name: String, isBlack: Boolean) = {
    val sheep = new Sheep(name)
    sheep.isBlack = isBlack
    sheep
  }

  // Factory method #2
  def apply(name: String) = new Sheep(name)
}

Sheep("Dolly", true) // Calls factory method #1
Sheep("Daisy") // Calls factory method #2
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