java 使用默认值初始化 HashMap？
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Initializing a HashMap with default values?
提问by Talen Kylon
I'm implementing the A* search algorithm given here, https://en.wikipedia.org/wiki/A*_search_algorithm
我正在实施这里给出的 A* 搜索算法，https://en.wikipedia.org/wiki/A *_search_algorithm
This line indicates we need to initiliaze a map with the default values of INFINITY,
这一行表示我们需要使用 INFINITY 的默认值初始化一个映射，
gScore := map with default value of Infinity
So I tried that here,
所以我在这里尝试，
Map<State, Double> gScore = new HashMap<State, Double>(Double.POSITIVE_INFINITY);
This does not work however the following does;
这行不通，但以下方法行不通；
Map<State, Double> gScore = new HashMap<State, Double>((int) Double.POSITIVE_INFINITY);
I'm wondering why, and what impact (if any) it will have on my implementation.
我想知道为什么，以及它会对我的实现产生什么影响（如果有的话）。
采纳答案by Louis Wasserman
There is no way to initialize a map with a default value in Java, and your second version will not create a map with a default value of infinity, but instead will try to create an infinitely large map. (Not really, but it'll try creating the largest map possible.)
在 Java 中无法使用默认值初始化地图，您的第二个版本不会创建默认值为无穷大的地图，而是会尝试创建无限大的地图。（不是真的，但它会尝试创建尽可能大的地图。）
Instead, modify the algorithm: anytime you do map.get(key), check if the value is null and, if so, replace it with infinity.
相反，修改算法：无论何时map.get(key)，检查该值是否为空，如果是，则将其替换为无穷大。
回答by Stephen C
I concur with @Louis that the bestsolution is to check the result of the getcall to see if it is null.
我同意@Louis 的观点，即最好的解决方案是检查get调用的结果是否为null.
However, it is also possibleto create a subclass of HashMapwith an override for the getmethod that returns a default value if super.get(key)returns null. But beware of anomalies; e.g.
然而，也有可能产生的子类HashMap具有的覆写get返回如果默认值方法super.get(key)的回报null。但要注意异常情况；例如
Iterating the map will give you only the entries that are "really there".
If there is a real entry with a
nullvalue (because you calledput(key, null)) you won't get anullwhen you callgetfor the entries key. But but the entry willshow up in the iteration ... with valuenull.
迭代地图只会给你“真的存在”的条目。
如果有一个带有
null值的真实条目（因为您调用了put(key, null)），则null在您调用get条目键时将不会得到 a 。但是，该条目将出现在迭代中 ... 带有 valuenull。
So, from an OO design perspective, a better approach (to extending HashMap) would be to create an application-specific class that only exposes a subset of the Mapfunctionality.
因此，从 OO 设计的角度来看，更好的方法（扩展HashMap）是创建一个特定于应用程序的类，该类只公开Map功能的一个子集。
回答by Eran
Instead of explicitly putting Double.POSITIVE_INFINITYfor all the possible keys (which may be inefficient), you can use putIfAbsent.
相反的明确把Double.POSITIVE_INFINITY所有可能的密钥（这可能是低效的），你可以使用putIfAbsent。
In any place where you have:
在您拥有的任何地方：
value = map.get(key);
you can change it to:
您可以将其更改为：
value = map.putIfAbsent(key,Double.POSITIVE_INFINITY);
This will behave as if the value of any key not present in the Map(or any key whose current value is null) is Double.POSITIVE_INFINITY. It will put the value Double.POSITIVE_INFINITYin the Mapfor any such keys.
这将表现得好像 中不存在的Map任何键的值（或当前值为 的任何键null）是Double.POSITIVE_INFINITY。它会将值Double.POSITIVE_INFINITY放入Map任何此类键的 中。
It will save you the need to check if map.get(key)returns null.
它将使您无需检查是否map.get(key)返回null。
