实现HashSet

Question

在哈希集的实现中有一些奇怪的编译错误。这段代码

class HashSet[A] extends scala.collection.mutable.Set[A] {
  override def +=(elem: A): HashSet[A] = {
    /* doing stuff */
    this
  }
}

产生下列错误

Error:(15, 16) overriding method += in trait SetLike of type (elem: A)HashSet.this.type;
 method += has incompatible type
  override def +=(elem: A): HashSet[A] = {
               ^
Error:(15, 16) overriding method += in trait Builder of type (elem: A)HashSet.this.type;
 method += has incompatible type
  override def +=(elem: A): HashSet[A] = {
               ^
Error:(15, 16) overriding method += in trait Growable of type (elem: A)HashSet.this.type;
 method += has incompatible type
  override def +=(elem: A): HashSet[A] = {
               ^

我怎么才能治好他们？

Answer 1

要重写的方法是HashSet.+=，它具有以下定义：

def +=(elem: A): HashSet.this.type

返回类型是HashSet.this.type。您正在用一个返回不同类型( HashSet[A] )的方法重写它。这是不允许的

换句话说:突变一个可变集合不应该仅仅返回任何相同类型的集合，它应该返回完全相同的集合(即this)。在方法中正确地返回this，但是可以使返回类型更加精确，显式地声明您不仅返回任何HashSet，还返回此HashSet。

class HashSet[A] extends scala.collection.mutable.Set[A] {
  override def +=(elem: A):       this.type   = ???
  override def iterator:          Iterator[A] = ???
  override def -=(elem: A):       this.type   = ???
  override def contains(elem: A): Boolean     = ???
}

由于您正在使用返回类型this.type重写方法，因此只需让Scala推断返回类型也会有效：

class HashSet[A] extends scala.collection.mutable.Set[A] {
  override def +=(elem: A)                    = ???
  override def iterator                       = ???
  override def -=(elem: A)                    = ???
  override def contains(elem: A)              = ???
}