Scala教程10 Trait

Trait
Scala中，trait和Java中的接口类似，不同的地方是，trait中可以包含实体方法。定义方式如下:
trait traitName {....} 比如:

scala> trait Talk {
     |  def hello(name:String) = println("Hello,"+name)
     | }
defined trait Talk

Scala中，class可以添加trait；如果class没有父类，则用extends 添加trait，否则用with；class可以直接使用被添加的trait中的方法，示例如下：

scala> class People extends Talk
defined class People

scala> val peple = new People()
peple: People = People@1258b446

scala> peple.hello("Alice")
Hello,Alice

添加多个trait，示例如下：
scala> trait Talk {
     |  def hello(name:String) = println("Hello,"+name)
     | }
defined trait Talk

scala> trait Run {
     |  def run() = println("I can run")
     | }
defined trait Run

scala> class Animal
defined class Animal

scala> class People extends Animal with Run with Talk
defined class People

scala> val people = new People()
people: People = People@72215167

scala> people.run
I can run

scala> people.hello("Alice")
Hello,Alice

trait可以继承class，也可以添加其它trait，示例如下：
scala> class Bird
defined class Bird

scala> trait Sing extends Bird {
     |  def sing() = println("A bird can sing")
     | }
defined trait Sing

scala> trait Walk
defined trait Walk

scala> trait SingAndWalk extends Sing with Walk
defined trait SingAndWalk

一旦一个trait继承了class，那么能添加该trait的只有该class或者其子类，示例如下：
scala> class Bird
defined class Bird

scala> trait BirdTrait extends Bird
defined trait BirdTrait

scala> class Parrot extends Bird
defined class Parrot

scala> class Deer
defined class Deer

scala> val bird = new Bird with BirdTrait
bird: Bird with BirdTrait = $anon$1@6c46215

scala> val parrot = new Parrot with BirdTrait
parrot: Parrot with BirdTrait = $anon$1@6f2e17a2

scala> val deer = new Deer with BirdTrait
:10: error: illegal inheritance; superclass Deer
 is not a subclass of the superclass Bird
 of the mixin trait BirdTrait
       val deer = new Deer with BirdTrait
                                ^

参考自：Programming Scala 2nd Edition