generics - 扩展 Scala 集合

Question

我想要一个 Map 试图覆盖现有键的值。我试过:

trait Unoverwriteable[A, B] extends scala.collection.Map[A, B] {
    case class KeyAlreadyExistsException(e: String) extends Exception(e)

    abstract override def + [B1 >: B] (kv: (A, B1)): Unoverwriteable[A, B1] = {
        if (this contains(kv _1)) throw new KeyAlreadyExistsException(
            "key already exists in WritableOnce map: %s".format((kv _1) toString)
        )
        super.+(kv)
    }

    abstract override def get(key: A): Option[B] = super.get(key)
    abstract override def iterator: Iterator[(A, B)] = super.iterator
    abstract override def -(key: A): Unoverwriteable[A, B] = super.-(key)
}

并得到:

<console>:11: error: type mismatch;
 found   : scala.collection.Map[A,B1]
 required: Unoverwirteable[A,B1]
               super.+(kv)
                      ^
<console>:16: error: type mismatch;
 found   : scala.collection.Map[A,B]
 required: Unoverwirteable[A,B]
           abstract override def -(key: A): Unoverwirteable[A, B] = super.-(key)
                                                                           ^

我对 Scala 很陌生，无法找到克服这个问题的方法。有什么帮助吗？ :)

编辑:我正在使用 Scala 2.8.0.Beta1-prerelease(这给 scala.collection 带来了一些变化)

Answer 1

这修复了您的编译错误:

trait Unoverwriteable[A, B] extends scala.collection.Map[A, B] {
    case class KeyAlreadyExistsException(e: String) extends Exception(e)

    abstract override def + [B1 >: B] (kv: (A, B1)): scala.collection.Map[A, B1] = {
        if (this contains(kv _1)) throw new KeyAlreadyExistsException(
            "key already exists in WritableOnce map: %s".format((kv _1) toString)
        )
        super.+[B1](kv)
    }

    abstract override def get(key: A): Option[B] = super.get(key)
    abstract override def iterator: Iterator[(A, B)] = super.iterator
    abstract override def -(key: A): scala.collection.Map[A, B] = super.-(key)
}

不过，我觉得你真的很想装饰 collection.mutable.Map#+= ， 如下:

trait Unoverwriteable[A, B] extends collection.mutable.Map[A, B] {
  case class KeyAlreadyExistsException(e: String) extends Exception(e)

  abstract override def +=(kv: (A, B)): this.type = {
    if (this contains (kv _1))
      throw new KeyAlreadyExistsException("key already exists in WritableOnce map: %s".format((kv _1) toString))
    super.+=(kv)
  }
}