Traits and inheritance

Scala provides two structures for inheritance. Classes (abstract or not) and traits. Traits are very similar to Ruby Mixins meaning that they can contain code like abstract classes but like interfaces multiple traits can be inherited from.
Like interfaces traits cannot have constructors but in Scala variables can be abstract and therefore provide an easy way to simulate a constructor.
There are no method resolution conflicts because method definitions are always resolved right to left:

1. class X extends Y with A with B with C

If ABC and Y all have the method (doit) the method in C will be used. If C calls super.doit that will call B.doit and so on.
Note: If Y defines doit the A, B, and C must define doit with the override keyword:

1. override def doit() = {...}

In the above example ABC must be traits but Y can be a class or a trait. When inheriting you must always have one extends keyword which can optionally followed by one or more with clauses.

1. scala> abstract class Animal {
2.      |  val legs:Int
3.      | val noise:String
4.      | def makeNoise() = println(noise)
5.      | }
6. defined class Animal
8. scala>  trait Quadriped {
9.      | self:Animal =>
10.      | val legs = 4
11.      | }
12. defined trait Quadriped
14. scala> trait Biped {
15.      | self:Animal =>
16.      | val legs = 2
17.      | }
18. defined trait Biped
20. scala> class Dog extends Animal with Quadriped {
21.      | val noise = "Woof"
22.      | override def makeNoise() = println( noise+" "+noise)
23.      | }
24. defined class Dog
26. scala> new Dog().makeNoise()
27. Woof Woof
29. scala> abstract class GenericAnimal extends Animal{ 
30.      | val noise = "glup"                          
31.      | }
32. defined class GenericAnimal
34. scala> val quad = new GenericAnimal() with Quadriped
35. quad: GenericAnimal with Quadriped = $anon$1@10bfb545
37. scala> quad.makeNoise()
38. glup
40. scala> val biped = new GenericAnimal() with Biped
41. biped: GenericAnimal with Biped = $anon$1@7669521
43. scala> val biped = new GenericAnimal() with Biped{
44.      | override val noise = "Hello"
45.      | }
46. biped: GenericAnimal with Biped = $anon$1@6366ce5f
48. scala> biped.makeNoise()
49. Hello