trait

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trait

老实说,感觉trait和Java中的interface已经很像了,因此下面我们主要说不像的.

1. Java用implements来实现接口, scala用extends和with来实现接口

   先看一个经典的有关日志的例子

   object TraitAddExtra {
     def main(args: Array[String]): Unit = {
       val savingsAccount = new SavingsAccount1
       savingsAccount.deposit(1000)
       savingsAccount.withDraw(300)
       savingsAccount.withDraw(800)
     }
   }
   
   trait MyLogger1 {
     def log(msg: String)
   
     def info(msg: String) = log("INFO\t" + msg)
   
     def warn(msg: String) = log("WARN\t" + msg)
   
     def err(msg: String) = log("ERR\t" + msg)
   }
   
   class SavingsAccount1 extends Acount(0, 0) with MyLogger1 {
     def withDraw(amount: Double): Unit = {
       if (amount > balance) err("Insufficient funds")
       else {
         balance -= amount
         info("withDraw money" + amount)
         warn("remained money" + balance)
       }
     }
   
     override def log(msg: String): Unit = println(msg)
   }
   

   class Acount (val id: Int, initialBalance: Double) {
     var balance = initialBalance
   
     def deposit(amount: Double): Double = {
       balance += amount
       balance
     }
   
     override def toString: String = id + "->" + balance
   }
   
   object Acount {
     private var lastNumber = 0
     println("i am in constructor of object")
   
     def newUniqueNumber(): Int = {
       lastNumber += 1
       lastNumber
     }
   
     def apply(initialBalance: Double): Acount = new Acount(newUniqueNumber(), initialBalance)
   
     def main(args: Array[String]): Unit = {
       val accounts = ArrayBuffer[Acount](Acount(100.0), Acount(1000.0), Acount(10000.0))
       accounts.foreach(println)
     }
   }
   

   • 通过以上代码我们发现,scala在这点上和java上没有多大区别,接口里都可以定义抽象方法和具体方法.

2. 多trait引入的处理--scala这里采用从最后一个开始倒着处理.(这个说明一下,这种功能java无法实现,无论你用interface还是abstract,均无法调用super.log()这个方法,故无法完成,其实Java里这里采用策略设计模式会去实现,但是也非常不好用,还得靠组合函数去实现.这里就体现出了scala的优势,确实非常舒服!)

   object TraitWhenOverlying extends App {
     //先调用ShortLogger 调用super.log为TimestampLogger调用super.log为PrintLogger
     val act1 = new SavingsAccount with PrintLogger with TimestampLogger with ShortLogger
     //先调用TimestampLogger 调用ShortLogger调用super..log为PrintLogger
     val act2 = new SavingsAccount with PrintLogger with ShortLogger with TimestampLogger
     //如果你想指定调用特定的trait,用super[Trait].log()
     //  val act3 = new SavingsAccount with LongLogger with PrintLogger
     act1.deposit(1000)
     act1.withDraw(400)
     act1.withDraw(700)
     act2.deposit(1000)
     act2.withDraw(400)
     act2.withDraw(700)
     /*act3.deposit(1000)
     act3.withDraw(400)
     act3.withDraw(700)*/
   }
   
   trait TimestampLogger extends PrintLogger {
     override def log(msg: String): Unit = {
       println("我被调用了,我在TimestampLogger里")
       super.log(new Date() + "\t" + msg)
     }
   }
   
   trait ShortLogger extends PrintLogger {
     val maxLength = 15
   
     override def log(msg: String): Unit = {
       println("我被调用了,我在ShortLogger里")
       super.log(if (msg.length <= maxLength) msg else msg.substring(0, maxLength - 3) + "...")
     }
   }
   

   结果

   我被调用了,我在ShortLogger里
   我被调用了,我在TimestampLogger里
   Tue Jan 09 16:42:16 CST 2018	Insufficient...
   我被调用了,我在TimestampLogger里
   我被调用了,我在ShortLogger里
   Tue Jan 09 1...
   

   ​

3. trait的初始化顺序:注意和上面的super.log比较,会发现方向正好相反,初始化是按照顺序正常构造,super是从右向左来调用,前者的顺序化是线性化的反向,后者的顺序是线性化的方向.

   • 首先调用超类的构造器
   • trait构造器在超类构造器之后,在类构造器之后.
   • trait构造器由左到右被构造
   • 每个trait中,父构造器先被构造
   • 如果多个trait共有一个父trait,而那个父trait已经被构造,则不会被再次构造
   • 所有trait构造完毕,类被构造

   ​

   以class SavingAcount extends Acount with FileLogger with ShortLogger为例

   • Acount(超类)
   • Logger(第一个trait的父trait)
   • ShortLogger(第一个trait)
   • FileLogger(第二个trait)
   • SavingAcount(类)

4. trait和类之间的唯一技术差别就是trait不能右构造器,只有一个默认的无参构造器.你应该记得object也是这样,不能有构造器,其他的特性都有.

5. trait原理

   • trait会被翻译成为JVM的类和接口

   • 抽象方法翻译为接口的抽象方法

   • 具体方法翻译为Java的默认方法

   • 如果有字段:

     • 翻译成接口就有getter,setter方法,其实出国初始化了的化,还会翻译成一个init方法

     • 如果有类继承这个trait,则会此类会获得这个字段,相当于java里的private ...

       ​

转载于:https://my.oschina.net/xd03122049/blog/1605327