Over a million developers have joined DZone.

Scala Wonderland: Semicolons, Singletons and Companion Objects

· Java Zone

Discover how AppDynamics steps in to upgrade your performance game and prevent your enterprise from these top 10 Java performance problems, brought to you in partnership with AppDynamics.

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

The Java Zone is brought to you in partnership with AppDynamics. AppDynamics helps you gain the fundamentals behind application performance, and implement best practices so you can proactively analyze and act on performance problems as they arise, and more specifically with your Java applications. Start a Free Trial.

Topics:

Published at DZone with permission of Rado Buranský , DZone MVB .

Opinions expressed by DZone contributors are their own.

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}