通过延时绑定混入类的方法,可以有效的回避这一点。 在trait里调用super可能解析成另一个trait的方法也可能解析成混入类的方法。
做一下名词解释,所谓延时绑定也叫动态绑定,发生在运行期;与之相对的静态绑定发生在编译器,熟悉C++的可以参考这篇文章对这两个概念有更深的了解: http://www.oschina.net/question/54100_20313。熟悉java的可以参考这篇文章: http://blog.csdn.net/zhangjk1993/article/details/24066085。
接下去,我们队scala中的 延时绑定进行详细的介绍。举例如下:
写个抽象类Writer:
abstract class Writer{ def writeMessage(message:String) }
任何继承Writer类的类都要实现writeMessage()方法,并且用super调用这个抽象方法,Scala会要求将方法申明为abstract override。有两层意思:
第一:override告诉scala,要为基类的一个已知方法提供一个实现。
第二:这个方法最后的“终极”实现由混入这个trait的类提供。
trait UpperCaseWriter extends Writer{ abstract override def writeMessage(message:String)=super.writeMessage(message.toUpperCase) } trait ProfanityFilterdWriter extends Writer{ abstract override def writeMessage(message:String)=super.writerMessage(message.replace("stupid","s_______")) }
使用super.writeMessage,Scala做了两件事:
首先:对这个调用进行了延时绑定
其次:要求混入这些类的特质(trait)提供该方法的实现。
上面的ProfanityFilteredWriter只负责处理有些粗鲁的单词
类StringWriterDelegate继承自抽象类Writer,将写消息的操作委托给一个StringWriter
class StringWriterDelegate extend Writer{ val writer=new java.io.StringWriter def writeMessage(message:String)=writer.write(message) override def toString():String=writer.toString() }
StringWriterDelegate可以混入一个或多个trait.
val myWriterProfanityFirst=new StringWriterDelegate with UpperCaseWriter with ProfanityFilteredWriter val myWriterProfanityLast=new StringWriterDelegate with ProfanityFilteredWriter with UpperCaseWriter myWriterProfanityFirst.writerMessage("There is no sin except stupidity") //输出:THERE IS NO SIN EXCEPT S____ITY //调用顺序说明 myWriterProfanityFirst->ProfanityFilterWriter-->UpperCaseWriter-->StringWriterDelegate ->writerMessage() myWriterProfanityLast.writerMessage("there us no sin except stupidity") //输出:THERE IS NO SIN EXCEPT STUPIDITY //调用顺序说明 myWriterProfanityLast->ProfanityFilterWriter-->UpperCaseWriter-->StringWriterDelegate ->writerMessage()