scala - HashSet 和 Set 有什么区别，应该什么时候使用？

Question

HashSet有什么区别和 Set什么时候应该使用每一种？这是Map对比 HashMap :

val hashSet = HashSet("Tomatoes", "Chilies")
val set = Set("Tomatoes", "Chilies")
set == hashSet // res: Boolean = true

Answer 1

Set是一种特质。您可以创建 Set 的实例通过调用 apply其伴生对象的方法，该方法返回默认的不可变实例 Set .例如:

val defaultSet = Set("A", "B")

HashSet是 Set 的具体实现可以实例化如下:

val hashSet = HashSet("A", "B")

看看“Scala 编程”中的引用，它解释了各种实现之间的差异:

The scala.collection.mutable.Set() factory method, for example, returns a scala.collection.mutable.HashSet, which uses a hash table internally. Similarly, the scala.collection.mutable.Map() factory returns a scala.collection.mutable.HashMap.

The story for immutable sets and maps is a bit more involved. The class returned by the scala.collection.immutable.Set() factory method, for example, depends on how many elements you pass to it, as shown in the table below. For sets with fewer than five elements, a special class devoted exclusively to sets of each particular size is used, to maximize performance. Once you request a set that has five or more elements in it, however, the factory method will return an implementation that uses hash tries.

Number of elements  Implementation
0                   scala.collection.immutable.EmptySet
1                   scala.collection.immutable.Set1
2                   scala.collection.immutable.Set2
3                   scala.collection.immutable.Set3
4                   scala.collection.immutable.Set4
5 or more           scala.collection.immutable.HashSet

这意味着对于不可变的 Set对于 5 个或更多元素，您的两个调用都应返回相同 Set 的实例子类。
Map 也是如此s。见 this关联。