Merge pull request #978 from som-snytt/review/singleton-objects

rewrote singleton objects tour

Author: SethTisue

_tour/singleton-objects.md

@@ -10,67 +10,101 @@ num: 13
 
 next-page: regular-expression-patterns
 previous-page: pattern-matching
-
 redirect_from: "/tutorials/tour/singleton-objects.html"
+prerequisite-knowledge: classes, methods, private-methods, packages, option
 ---
+An object is a class that has exactly one instance. It is created lazily when it is referenced, like a lazy val.
 
-Methods and values that aren't associated with individual instances of a [class](classes.html) belong in *singleton objects*, denoted by using the keyword `object` instead of `class`.
+As a top-level value, an object is a singleton.
 
+As a member of an enclosing class or as a local value, it behaves exactly like a lazy val.
+# Defining a singleton object
+An object is a value. The definition of an object looks like a class, but uses the keyword `object`:
+```tut
+object Box
 ```
-package test
 
-object Blah {
-  def sum(l: List[Int]): Int = l.sum
-}
+Here's an example of an object with a method:
 ```
+package logging
 
-This `sum` method is available globally, and can be referred to, or imported, as `test.Blah.sum`.
+object Logger {
+  def info(message: String): Unit = println(s"INFO: $message")
+}
+```
+The method `info` can be imported from anywhere in the program. Creating utility methods like this is a common use case for singleton objects.
 
-Singleton objects are sort of a shorthand for defining a single-use class, which can't directly be instantiated, and a `val` member at the point of definition of the `object`, with the same name. Indeed, like `val`s, singleton objects can be defined as members of a [trait](traits.html) or class, though this is atypical.
+Let's see how to use `info` in another package:
 
-A singleton object can extend classes and traits. In fact, a [case class](case-classes.html) with no [type parameters](generic-classes.html) will by default create a singleton object of the same name, with a [`Function*`](http://www.scala-lang.org/api/current/scala/Function1.html) trait implemented.
+```
+import logging.Logger.info
 
-## Companions ##
+class Project(name: String, daysToComplete: Int)
 
-Most singleton objects do not stand alone, but instead are associated with a class of the same name. The “singleton object of the same name” of a case class, mentioned above, is an example of this. When this happens, the singleton object is called the *companion object* of the class, and the class is called the *companion class* of the object.
+class Test {
+  val project1 = new Project("TPS Reports", 1)
+  val project2 = new Project("Website redesign", 5)
+  info("Created projects")  // Prints "INFO: Created projects"
+}
+```
 
-[Scaladoc]({{ site.baseurl }}/style/scaladoc.html) has special support for jumping between a class and its companion: if the big “C” or “O” circle has its edge folded up at the bottom, you can click the circle to jump to the companion.
+The `info` method is visible because of the import statement, `import logging.Logger.info`.
 
-A class and its companion object, if any, must be defined in the same source file. Like this:
+Imports require a "stable path" to the imported symbol, and an object is a stable path.
 
-```tut
-class IntPair(val x: Int, val y: Int)
+Note: If an `object` is not top-level but is nested in another class or object, then the object is "path-dependent" like any other member. This means that given two kinds of beverages, `class Milk` and `class OrangeJuice`, a class member `object NutritionInfo` "depends" on the enclosing instance, either milk or orange juice. `milk.NutritionInfo` is entirely distinct from `oj.NutritionInfo`.
 
-object IntPair {
-  import math.Ordering
+## Companion objects
 
-  implicit def ipord: Ordering[IntPair] =
-    Ordering.by(ip => (ip.x, ip.y))
-}
+An object with the same name as a class is called a _companion object_. Conversely, the class is the object's companion class. A companion class or object can access the private members of its companion. Use a companion object for methods and values which are not specific to instances of the companion class.
 ```
+import scala.math._
 
-It's common to see typeclass instances as [implicit values](implicit-parameters.html), such as `ipord` above, defined in the companion, when following the typeclass pattern. This is because the companion's members are included in the default implicit search for related values.
-
-## Notes for Java programmers ##
+case class Circle(radius: Double) {
+  import Circle._
+  def area: Double = calculateArea(radius)
+}
 
-`static` is not a keyword in Scala. Instead, all members that would be static, including classes, should go in a singleton object. They can be referred to with the same syntax, imported piecemeal or as a group, and so on.
+object Circle {
+  private def calculateArea(radius: Double): Double = Pi * pow(radius, 2.0)
+}
 
-Frequently, Java programmers define static members, perhaps `private`, as implementation aids for their instance members. These move to the companion, too; a common pattern is to import the companion object's members in the class, like so:
+val circle1 = new Circle(5.0)
 
+circle1.area
 ```
-class X {
-  import X._
 
-  def blah = foo
+The `class Circle` has a member `area` which is specific to each instance, and the singleton `object Circle` has a method `calculateArea` which is available to every instance.
+
+The companion object can also contain factory methods:
+```tut
+class Email(val username: String, val domainName: String)
+
+object Email {
+  def fromString(emailString: String): Option[Email] = {
+    emailString.split('@') match {
+      case Array(a, b) => Some(new Email(a, b))
+      case _ => None
+    }
+  }
 }
 
-object X {
-  private def foo = 42
+val scalaCenterEmail = Email.fromString("[email protected]")
+scalaCenterEmail match {
+  case Some(email) => println(
+    s"""Registered an email
+       |Username: ${email.username}
+       |Domain name: ${email.domainName}
+     """)
+  case None => println("Error: could not parse email")
 }
 ```
+The `object Email` contains a factory `fromString` which creates an `Email` instance from a String. We return it as an `Option[Email]` in case of parsing errors.
 
-This illustrates another feature: in the context of `private`, a class and its companion are friends. `object X` can access private members of `class X`, and vice versa. To make a member *really* private to one or the other, use `private[this]`.
+Note: If a class or object has a companion, both must be defined in the same file. To define companions in the REPL, either define them on the same line or enter `:paste` mode.
+
+## Notes for Java programmers ##
 
-For Java convenience, methods, including `var`s and `val`s, defined directly in a singleton object also have a static method defined in the companion class, called a *static forwarder*. Other members are accessible via the `X$.MODULE$` static field for `object X`.
+`static` members in Java are modeled as ordinary members of a companion object in Scala.
 
-If you move everything to a companion object and find that all you have left is a class you don't want to be able to instantiate, simply delete the class. Static forwarders will still be created.
+When using a companion object from Java code, the members will be defined in a companion class with a `static` modifier. This is called _static forwarding_. It occurs even if you haven't defined a companion class yourself.