当通过Map.put（K，V）添加一个值时，是否必须通过Map.get（K）返回相同的实例？ [英] When a value is added via Map.put(K, V), must the same instance be returned via Map.get(K)?

问题描述

假设您有以下代码：

 地图< Foo， map = new HashMap< Foo，Bar>（）; 
 
 Foo foo = new Foo（）; 
 Bar bar = new Bar（）; 
 map.put（foo，bar）; 
 
 Bar barReturned = map.get（foo）; 
  

Java是否需要 barReturned == bar ？也就是说，Java 需要 barReturned 与 bar 相同的实例 c $ c>？如果没有，需要什么语义？

Javadoc 建议 barReturned == bar 必须为true，但我不能100％ p>

V get（Object key）

返回指定键映射的值，如果此映射不包含键的映射，则返回 null 。

更正式地说，如果此映射包含从 k 到值 v ，使得（key == null？k == null：key.equals（k）），则此方法返回 v ;否则返回 null 。 （最多只能有一个这样的映射。）

如果此映射允许 null 值， of null不一定 表示该映射不包含该键的映射;还有可能映射将键明确映射到 null 。 containsKey操作可用于区分这两种情况。

参数：

key - 要返回其关联值的键

/ p>

或 null 如果此地图不包含键的映射

（强调我）

编辑：我理解，与标准库捆绑的 Map 的实现遵循 barReturned == bar 语义。我想知道的是这个行为是否必需根据文档。例如，如果我编写自己的实现 Map ？

的解决方案，我还必须遵守这些语义方案

如果你问是否可以打破这种关系，我认为答案是是。例如，如果您正在实现一个类似持久高速缓存的映射，那么特定值可能会写入磁盘，如果在一段时间内未使用，则稍后重新加载。你在这种情况下不会有参考平等，但是没关系。显然，你想要记录与标准行为的任何偏差，但我不认为这是一个合理使用地图的范围。

Suppose you have this code:

Map<Foo, Bar> map = new HashMap<Foo, Bar>();

Foo foo = new Foo();
Bar bar = new Bar();
map.put(foo, bar);

Bar barReturned = map.get(foo);

Does Java require that barReturned == bar? That is, does Java require that barReturned be the same instance as bar? If not, what semantics are expected?

The Javadoc suggests that barReturned == bar must be true, but I'm not 100% sure:

V get(Object key)

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key k to a value v such that (key==null ? k==null : key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

If this map permits null values, then a return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

Parameters:

key - the key whose associated value is to be returned

Returns:

THE VALUE TO WHICH THE SPECIFIED KEY IS MAPPED, or null if this map contains no mapping for the key

(Emphasis mine)

Edit: I understand that the implementations of Map that come bundled with the standard library adhere to the barReturned == bar semantics. What I want to know is whether or not this behavior is required as per the documentation. For example, must I also adhere to these semantics if I write my own class that implements Map?

解决方案

If you are asking whether you can break that relation, i think the answer is "yes". for instance, if you were implementing a Map which acted like a persistent cache, a specific value may be written to disk if not used in a while, then reloaded later. you will not have reference equality in that situation, but that's okay. obviously, you would want to document any deviations from standard behavior, but i don't think this is out of the realm of a reasonable usage of a Map.

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