ConcurrentHashMap的：我们能相信它？ [英] ConcurrentHashMap: Can we trust on it?
问题描述
从ConcurrentHashMap中的文件：
From the document of ConcurrentHashMap:
一个哈希表支持检索和更新的所期望可调整并发的完全并发。
A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates.
我们能否完全相信的ConcurrentHashMap 并线程安全的操作？
Can we fully believe that ConcurrentHashMap does thread safe operation?
我使用的ConcurrentHashMap与他们values.My键值对映射的关键是：
I am using ConcurrentHashMap for mapping key with their values.My key-value pair is:
Map<Integer,ArrayList<Double>> map1 = new ConcurrentHashMap();
键范围从[0,1000000]的大小。我有可以一次访问/修改对应值到钥匙20个线程。这不是那么频繁，但该条件是可能的。我是
从下面的方法获得一个无边：
The size of key ranges from [0,1000000]. I have 20 threads which can access/modify value corresponding to a key at a time. This not so frequent but that condition is possible. I am getting an infinity from following method:
Double sum =0.0;
sum = sum + Math.exp(getScore(contextFeatureVector,entry.getValue())+constant);
contextFeatureVector 和 entry.getValue（）是与键关联数组列表。
contextFeatureVector and entry.getValue()are arraylist associated with a key.
constant =0.0001
private double getScore(List<Double> featureVector,List<Double>weightVector) throws NullPointerException
{
double score =0.0;
int length = featureVector.size();
for (int i =0 ; i< length ; i++){
score = score + (featureVector.get(i)*weightVector.get(i));
}
return score;
}
两者 featureVector＆LT;>和 weightVector 看起来
[-0.005554038592516575, 0.0048966974158881175, -0.05315976588195846, -0.030837804373964654, 0.014483064988148562, -0.018962129117649, -0.015221386014208877, 0.015825702365331477, -0.11363620479662287, 0.00802609847263844, -0.062106636476812194, 0.008108854471293185, -0.03193255218671684, 0.04949650992670292, -0.0545583154094599, -0.04873314092706468, 0.013534731656877033, 0.08433117163682455, 0.050310355477044114, -0.002420513353516017, -0.02708299928442614, -0.023489187394176294, -0.1277699782685597, -0.10071004855129333, 0.08649040730064464, -0.04940329664431305, -0.027481729446035053, -0.0571846057609884, -0.036738550618481455, -0.035608113682344365]
因此，从getScore返回不下去格外太大的价值。这将是在
几千。
thus the value returned from getScore does not go exceptionally too large. it will be in some thousands.
推荐答案
您使用的数据结构，使我相信一定有你的code一些bug。最有可能你是从地图获取列表，并更新它：
The data structure you use makes me believe there must some bug in your code. Most likely you are fetching the list from map and updating it:
map1.get(42).add(5);
注意添加（5）是不会线程安全的，因为它在普通操作的ArrayList 。您也需要线程安全的的ArrayList 或<一个href=\"http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ConcurrentHashMap.html#replace%28K,%20V,%20V%29\"相对=nofollow> 更换（K键，V属性oldValue，V newValue）以 方法。
Note that add(5) is not thread-safe as it operates on ordinary ArrayList. You either need thread safe ArrayList or replace(K key, V oldValue, V newValue) method.
如果您通过的ConcurrentHashMap 正在给担保仔细阅读，就可以有效地使用它。
If you read carefully through the guarantees ConcurrentHashMap is giving, you can use it effectively.
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