集合视图是什么？ [英] What is a view of a collection?

问题描述

在使用番石榴时，我已经读过几次 view 。 a>收集并阅读其文档。

I've been reading the term view a few times when using Guava collections and reading its documentation.

我一直在寻找一种解释，说明在这种情况下视图是什么以及它是否在Guava之外使用。在此处，它经常被使用。此类型名称中有 view 。

I've looked for an explanation of what a view is in this context and whether it's a term used outside of Guava. It's quite often used here. This type from Guava has view in its name.

我的猜测是，一个集合的视图是另一个具有相同数据但结构化的集合不同地例如，当我将条目从 java.util.HashSet 添加到 java.util.LinkedHashSet 时，后者是前者的看法。

My guess is that a view of a collection is another collection with the same data but structured differently; for instance when I add the entries from a java.util.HashSet to a java.util.LinkedHashSet the latter would be a view of the former. Is that correct?

如果有的话，有人可以用指向 view 的公认定义的链接吸引我吗？

Can somebody hook me up with a link to an accepted definition of view, if there is one?

谢谢。

推荐答案

另一个对象的视图根本不包含自己的数据。它的所有操作都是根据对另一个对象的操作实现的。

A view of another object doesn't contain its own data at all. All of its operations are implemented in terms of operations on the other object.

例如， keySet（） 地图的视图可能具有如下所示的实现：

For example, the keySet() view of a Map might have an implementation that looks something like this:

class KeySet implements Set<K> {
  private final Map<K, V> map;

  public boolean contains(Object o) {
    return map.containsKey(o);
  }

  ...
}

特别是，只要您修改视图的后备对象-在这里， Map 后退 keySet（）-视图反映了相同的更改。例如，如果调用 map.remove（key），则 keySet.contains（key）将返回 false ，而无需执行其他任何操作。

In particular, whenever you modify the backing object of your view -- here, the Map backs the keySet() -- the view reflects the same changes. For example, if you call map.remove(key), then keySet.contains(key) will return false without you having to do anything else.

或者， Arrays.asList（array）提供该数组的列表视图。

Alternately, Arrays.asList(array) provides a List view of that array.

String[] strings = {"a", "b", "c"};
List<String> list = Arrays.asList(strings);
System.out.println(list.get(0)); // "a"
strings[0] = "d";
System.out.println(list.get(0)); // "d"
list.set(0, "e");
System.out.println(strings[0]); // "e"

视图只是查看原始支持对象中数据的另一种方式- Arrays.asList 允许您使用 List API访问普通数组； Map.keySet（）允许您访问 Map 的键，就好像它是完全普通的 Set -全部无需复制数据或创建其他数据结构。

A view is just another way of looking at the data in the original backing object -- Arrays.asList lets you use the List API to access a normal array; Map.keySet() lets you access the keys of a Map as if it were a perfectly ordinary Set -- all without copying the data or creating another data structure.

通常，使用视图而不是复制是效率。例如，如果您有一个数组，并且需要将其添加到采用 List 的方法中，则无需创建新的 ArrayList 和数据的完整副本- Arrays.asList 视图仅占用恒定的额外内存，并且仅实现所有通过访问原始数组来列出方法。

Generally, the advantage of using a view instead of making a copy is the efficiency. For example, if you have an array and you need to get it to a method that takes a List, you're not creating a new ArrayList and a whole copy of the data -- the Arrays.asList view takes only constant extra memory, and just implements all the List methods by accessing the original array.

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