Scala Cookbook

Introduction

The word “object” has a dual meaning in Scala. As with Java, you use it to refer to an instance of a class, but in Scala, object is also a keyword.

The first three recipes in this chapter look at an object as an instance of a class, show how to cast objects from one type to another, demonstrate the Scala equivalent of Java’s .class approach, and show how to determine the class of an object.

The remaining recipes demonstrate how the object keyword is used for other purposes. You’ll see how to use it to launch Scala applications and to create Singletons. There’s also a special type of object known as a package object. Using a package object is entirely optional, but it provides a nice little out-of-the-way place where you can put code that’s common to all classes and objects in a particular package level in your application. For instance, Scala’s root-level package object contains many lines of code like this:

type Throwable = java.lang.Throwable
type Exception = java.lang.Exception
type Error = java.lang.Error

type Seq[+A] = scala.collection.Seq[A]
val Seq = scala.collection.Seq

Declaring those type definitions in Scala’s root package object helps to make the rest of the code a little bit cleaner, and also keeps these definitions from cluttering up other files.

You’ll also see how to create a companion object to solve several problems. For instance, one use of a companion object is to create the equivalent of Java’s static members. You can also use a companion object so consumers ...