可变 Scala 集合的不可修改视图 [英] Unmodifiable view of a mutable Scala collection

问题描述

我有一个带有私有字段的类，它是一个可变集合.这个特定实例中的字段是 ArrayBuffer，尽管我的问题扩展到任何有限的、有序的、随机访问的集合类型.我想公开这个字段，不允许其他人修改它.在 Java 中，我会添加一个方法，如:

I have a class with a private field that is a mutable collection. The field in this particular instance is an ArrayBuffer, although my question extends to any finite, ordered, random-access collection type. I want to expose this field without permitting others to modify it. In Java I would add a method like:

private List<T> theList;

public List<T> getList() {
    return Collections.unmodifiableList(theList);
}

在 Java 中，我们只接受结果是一个 List，它没有完全实现 List 接口，因为 #add 和朋友抛出UnsupportedOperationException.

In Java we just accept that the result is a List that doesn't fully implement the List interface because #add and friends throw UnsupportedOperationException.

在 Scala 中，我希望找到一个合适的 trait 和访问器，如 iterator、size 和 apply(用于通过索引检索值) 但没有突变体.是否存在我还没有发现的这种类型?

In Scala, I would expect to find an appropriate trait with accessors like iterator, size, and apply (for retrieving values by index) but no mutators. Does such a type exist that I just haven't found yet?

推荐答案

Scala 集合库是面向不变性的，不变性不仅仅指的是不允许修改给定的集合，还包括您保证该集合不会被任何人修改.

The Scala collection library is oriented to immutability, and immutability doesn't refer only to the fact that you are not allowed to modify a given collection, but also that you are guaranteed that the collection won't be ever modified by anyone.

因此，您不能也不应该从 Scala 中的可变缓冲区中获取像 immutable.Seq 这样的集合作为视图，因为它违反了这一保证.

So you cannot and you shouldn't get a collection like immutable.Seq as a view from a mutable buffer in Scala since it breaks that guarantee.

但是您可以很容易地实现不可修改的可变 Seq 的概念，如下所示:

But you can implement the concept of unmodifiable mutable Seq easy enough like so:

class UnmodifiableSeq[A](buffer: mutable.Seq[A]) extends mutable.Seq[A]{
    def update(idx: Int, elem: A) {throw new UnsupportedOperationException()}

    def length = buffer.length

    def apply(idx: Int) = buffer(idx)

    def iterator = buffer.iterator
}

用法:

val xs = Array(1, 2, 3, 4)
val view = new UnmodifiableSeq(xs)
println(view(2)) >> 3
view(2) = 10 >> Exception in thread "main" java.lang.UnsupportedOperationException

获得不可修改的集合视图的一种可能更好的方法是向下转换到 collection.Seq，它不提供可变更新操作:

A probably better way of obtaining an unmodifiable view of the collection is by downcasting to collection.Seq which provides no mutable update operations:

val xs = Array(1, 2, 3)
val view: Seq[Int] = xs //this is unmodifiable

或者创建一个扩展 Seq 的包装器，如果您有自己的自定义可变类.

or creating a wrapper which extends Seq if you have your own custom mutable class.

class UnmodifiableView[A](col: MutableCollection[A]) extends collection.Seq[A]{
    def length = col.length

    def apply(idx: Int) = col(idx)

    def iterator = col.iterator
}

scala.collection.Seq trait 不保证任何不变性，但它也不允许任何修改操作，因此它看起来非常合适.

The scala.collection.Seq trait does not make any guarantees of immutability but it also does not allow any modifying operations so it seems the perfect fit.

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