为什么在哈希映射中使用空值或空键是有用的？

Question

Hashtable does not allow null keys or values, while HashMap allows null values and 1 null key.

Questions:

1. Why is this so?
2. How is it useful to have such a key and values in HashMap?

Answer 1

1. Why is this so?

HashMap is newer than Hashtable and fixes some of its limitations.

I can only guess what the designers were thinking, but here are my guesses:

• Hashtable calculates a hash for each key by calling hashCode on each key. This would fail if the key were null, so this could be a reason for disallowing nulls as keys.
• The method Hashtable.get returns null if the key is not present. If null were a valid value it would be ambiguous as to whether null meant that the key was present but had value null, or if the key was absent. Ambiguity is bad, so this could be a reason for disallowing nulls as values.

However it turns out that sometimes you do actually want to store nulls so the restrictions were removed in HashMap. The following warning was also included in the documentation for HashMap.get :

A return value of null does not necessarily indicate that the map contains no mapping for the key; it is also possible that the map explicitly maps the key to null.

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2. How is it useful to have such a key and values in HashMap?

It is useful to explicitly store null to distinguish between a key that you know exists but doesn't have an associated value and a key that doesn't exist. An example is a list of registered users and their birthdays. If you ask for a specific user's birthday you want to be able to distinguish between that user not existing and the user existing but they haven't entered their birthday.

I can't think of any (good) reason for wanting to store null as a key, and in general I'd advise against using null as a key, but presumably there is at least one person somewhere that needs that keys that can be null.

Answer 2

Well, I think Mark Byers answered perfectly, so just a simple example where null values and keys can be useful:

Imagine you have an expensive function that always returns the same result for the same input. A map is a simple way for caching its results. Maybe sometimes the function will return null, but you need to save it anyway, because the execution is expensive. So, null values must be stored. The same applies to null key if it's an accepted input for the function.

Answer 3

HashTable is very old class , from JDK 1.0.The classes which are in place from JDK 1.0 are called Legacy classes and by default they are synchronized .

To understand this, first of all you need to understand comments written on this class by author. “This class implements a hashtable, which maps keys to values. Any non-null object can be used as a key or as a value. To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method and the equals method.”

HashTable class is implemented on hashing mechanism, means to store any key-value pair, its required hash code of key object. HashTable calculates a hash for each key by calling hashCode on each key. This would fail If key would be null, it will not be able to give hash for null key, it will throw NullPointerException and similar is the case for value, throwing null if the value is null .

But later on it was realised that null key and value has its own importance, then revised implementations of HashTable were introduced like HashMap which allow one null key and multiple null values.

For HashMap, it allows one null key and there is a null check for keys, if the key is null then that element will be stored in a zero location in Entry array.

We cannot have more than one Null key in HashMap because Keys are unique therefor only one Null key and many Null values are allowed.

USE - Null key we can use for some default value.

Modified and better implementation of HashTable was later introduced as ConcurrentHashMap .

Answer 4

In addition to what answered by Mark Bayers,, Null is considered as data and it has to be stored as a value for further checking. In many cases null as value can be used to check if key entry is there but no value is assigned to it, so some action can be taken accordingly. This can be done by first checking if key is there, and then getting value. There is one more case in which just put whatever data is coming(without any check). All the checks are applied to it after getting it.

Whereas null as a key, i think can be used to define some default data. Usually null as a key does not make much sense.

Answer 5

HashMap爵士也在内部使用hashCode（）方法在HashMap中插入一个元素，所以我认为这不是“为什么HashTable允许空键”的正当理由

Answer 6

It will make the Map interface easier to use / less verbose. null is a legitimate value for reference types. Making the map able to handle null keys and values will eliminate the need for null checking before calling the api. So the map api create less "surprises" during runtime.

For example, it is common that map will be used to categorize a collection of homogeneous objects based a single field. When map is compatible with null, the code will be more concise as it is just a simple loop without any if statement (of course you will need to make sure the collection does not have null elements). Fewer lines of code with no branch / exception handling will be more likely to be logically correct.

On the other hand, not allowing null will not make the map interface better/safer/easier to use. It is not practical to rely on the map to reject nulls - that means exception will be thrown and you have to catch and handle it. Or, to get rid of the exception you will have to ensure nothing is null before calling the map methods - in which case you don't care if map accepts null since you filtered the input anyway.