Java HashMap在迭代时添加新条目
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Java HashMap add new entry while iterating
提问by Buru
In a HashMap
在 HashMap 中
map = new HashMap<String,String>();
it = map.entrySet().iterator();
while (it.hasNext())
{
entry = it.next();
it.remove(); //safely remove a entry
entry.setValue("new value"); //safely update current value
//how to put new entry set inside this map
//map.put(s1,s2); it throws a concurrent access exception
}
When i trying to add a new entry to map it throws ConcurrentModificationException. For remove and update iterator has safely removing methods. How to add new entry while iterating?
当我尝试添加一个新条目来映射时，它抛出ConcurrentModificationException. 对于删除和更新迭代器具有安全删除方法。如何在迭代时添加新条目？
回答by Dunes
You need to consider what it means to put a value to a Map whilst iterating. HashMap defines no order over which its entries will be iterated over. So when you put a new entry, should the entry be returned by the iterator later or not. Consistency of behaviour is important. However, whichever way you decide you'll get inconsistent behaviour when you put a new value to a preexisting key. If the key has already been iterated over then the change won't appear and will appear if the key has yet to be produced by the iterator.
您需要考虑在迭代时将值放入 Map 意味着什么。HashMap 没有定义其条目将被迭代的顺序。因此，当您放置新条目时，该条目是否应该稍后由迭代器返回。行为的一致性很重要。但是，无论您决定采用哪种方式，当您将新值放入预先存在的键时都会出现不一致的行为。如果该键已经被迭代，那么更改将不会出现，如果该键尚未由迭代器生成，则该更改将出现。
A simple way to overcome this problem is to create a temporary Map of the new key-value pairs and add the temporary Map to the main Map at the end of your iteration.
解决这个问题的一个简单方法是为新的键值对创建一个临时 Map，并在迭代结束时将临时 Map 添加到主 Map。
Map<String,String> values = ...
Map<String,String> temp = new HashMap<>();
for (Entry<String,String> entry : values.entrySet()) {
if ("some value".equals(entry.getValue()) {
temp.put(entry.getValue(), "another value");
}
}
values.putAll(temp);
回答by Juned Ahsan
You need to use ConcurrentHashMapto add elements while iterating the collection. HashMap uses fail-fast iterator, which throws ConcurrentModificationException when the collection is updated while iterating. Whereas ConcurrentHashMapuses fail-safe iterator, which basically works on the clone of the underlying collection and hence allows modification while iterating.
您需要ConcurrentHashMap在迭代集合时使用来添加元素。HashMap 使用快速失败迭代器，它在迭代时更新集合时抛出 ConcurrentModificationException。而ConcurrentHashMap使用故障安全迭代器，它基本上适用于底层集合的克隆，因此允许在迭代时进行修改。
回答by john16384
How about:
怎么样：
map = new HashMap<String,String>();
it = map.entrySet().iterator();
while (it.hasNext())
{
entry = it.next();
entry.setValue("new value"); // update current value
}
I checked the HashMap implementation, it does not change its modification count when updating an entry like this. I also see no reason why this shouldn't be allowed. Nothing is removed, nothing is added and no keys are changed.
我检查了 HashMap 实现，它在更新这样的条目时不会更改其修改计数。我也认为没有理由不允许这样做。没有删除任何内容，没有添加任何内容，也没有更改任何密钥。
回答by Bogdan Alexandru
What I did was to create an array with the current elements and then iterate through the array:
我所做的是用当前元素创建一个数组，然后遍历该数组：
Feature[] initialFeatures = featureMap.values().toArray(new Feature[featureMap.values().size()]);
for (Feature feature : initialFeatures)
{/* Code that can safely add to the featureMap */}
