问题描述
什么是等效的Scala构造函数（用于创建不可变 HashSet ）到Java
What is the equivalent Scala constructor (to create an immutable HashSet) to the Java
new HashSet<T>(c)
其中 c 的类型为 Collection<？扩展T> ？。
我可以在 HashSet中找到 对象是 apply 。
推荐答案
有两个部分回答。第一部分是采用T *的Scala变量参数方法是采用Seq [T]的方法。您告诉Scala使用seq：_ *将Seq [T]视为参数列表而不是单个参数。
There are two parts to the answer. The first part is that Scala variable argument methods that take a T* are a sugaring over methods taking Seq[T]. You tell Scala to treat a Seq[T] as a list of arguments instead of a single argument using "seq : _*".
第二部分是将Collection [T]转换为Seq [T]。在Scala的标准库中还没有通用的内置方法，但是一种非常简单（如果不一定有效）的方法是通过调用toArray。这是一个完整的例子。
The second part is converting a Collection[T] to a Seq[T]. There's no general built in way to do in Scala's standard libraries just yet, but one very easy (if not necessarily efficient) way to do it is by calling toArray. Here's a complete example.
scala> val lst : java.util.Collection[String] = new java.util.ArrayList
lst: java.util.Collection[String] = []
scala> lst add "hello"
res0: Boolean = true
scala> lst add "world"
res1: Boolean = true
scala> Set(lst.toArray : _*)
res2: scala.collection.immutable.Set[java.lang.Object] = Set(hello, world)
注意scala.Predef.Set和scala.collection.immutable.HashSet是同义词。
Note the scala.Predef.Set and scala.collection.immutable.HashSet are synonyms.
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