Daily scala

Scala does not have a enum keyword like java so enumerations are not quite as smooth. Depending on your requirements there are two ways to make enumerations.

1. Create an object that extends the Enumeration class
2. Create a case-object hierarchy.
   

I present both ways. Here are some tips:

• If you only need discrete and related values without custom behaviour create and object that extends Enumeration
• If each value has custom information associated with it use case-objects

In the following examples notice the use of the sealed keyword when defining the abstract class MyEnum. Sealed specifies that the heirarchy is sealed. Only classes defined in the same file can extend the class.

Also notice in the case object example that the enumeration values are "case object" not "case class". The two are similar except that there is only one instance of a case object. However all the same boiler plate code is generated and you can still match in the same way.

1. // Note: MyEnum is an object NOT a class
2. scala> object MyEnum extends Enumeration("one", "two", "three") {
3.      | type MyEnumType = Value
4.      | val One, Two, Three = Value
5.      | }
6. defined module MyEnum
7. scala> MyEnum.One
8. res1: MyEnum.Value = one
9. scala> def printEnum( value:MyEnum.MyEnumType ) = println( value.toString )
10. printEnum: (MyEnum.MyEnumType)Unit
11. scala> printEnum(MyEnum.Two)
12. two
13. // If you don't want to prefix enums with MyEnum. Then you
14. // can import the values.  This is similar to static imports in java
15. scala> import MyEnum._
16. import MyEnum._
17. scala> def printEnum( value:MyEnumType ) = println( value.toString )
18. printEnum: (MyEnum.MyEnumType)Unit
19. scala> printEnum(Three)
20. three
21. // Similar but with cases objects
22. // Notice MyEnum is 'sealed' and the parameters have the 'val' keyword so they are public
23. scala> abstract sealed class MyEnum(val name:String, val someNum:Int)
24. defined class MyEnum
25. scala> case object One extends MyEnum("one", 1)
26. defined module One
27. scala> case object Two extends MyEnum("two", 2)
28. defined module Two
29. scala> case object Three extends MyEnum("three", 3)
30. defined module Three
31. scala> def printEnum(value:MyEnum) = println(value.name, value.someNum)
32. printEnum: (MyEnum)Unit
33. scala> printEnum(One)
34. (one,1)

Scala does not have static methods in the same way that Java does. As a replacement Scala has "objects" and object is a singleton object whose methods can be called in the same manner one would call a static method in Java. The big differentiator is that objects are complete objects and can extent abstract classes and traits.

Objects are sometime referred to as modules as well. See next section for more on modules. In addition there is a special situation where a class has what is called a companion object. That is a topic for another day. Finally you can have case objects, also a topic for another day. case objects will be address as part of the Enumeration topic.

1. scala> abstract class SuperClass {
2.     | def method = "hi"
3.     | val value = 10
4.     | }
5. defined class SuperClass
6. scala> object MyObject extends SuperClass {
7.     | override def method = "Hello"
8.     | def anotherMethod = "other"
9.     | }
10. defined module MyObject
11. scala> MyObject.method
12. res0: java.lang.String = Hello
13. scala> MyObject.value
14. res1: Int = 10
15. scala> MyObject.anotherMethod
16. res2: java.lang.String = other

Objects are also a good way to modularize projects. You can define classes and other objects within an object

1. scala> object Outer {
2.     | case class Data(name:String)
3.     |
4.     | def print(data:Data) = Console.println(data.name)
5.     |
6.     | object Factory {
7.     | def defaultData = Data("defaultName")
8.     | }
9.     | }
10. defined module Outer
11. scala> val data = Outer.Factory.defaultData
12. data: Outer.Data = Data(defaultName)
13. scala> Outer.print(data)
14. defaultName