在 Java 中修改对象时如何迭代它？
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How can I iterate over an object while modifying it in Java?
提问by Hristo
Possible Duplicates:
Java: Efficient Equivalent to Removing while Iterating a Collection
Removing items from a collection in java while iterating over it
I'm trying to loop through HashMap:
我正在尝试循环HashMap：
Map<String, Integer> group0 = new HashMap<String, Integer>();
... and extract every element in group0. This is my approach:
... 并提取group0. 这是我的方法：
// iterate through all Members in group 0 that have not been assigned yet
for (Map.Entry<String, Integer> entry : group0.entrySet()) {
// determine where to assign 'entry'
iEntryGroup = hasBeenAccusedByGroup(entry.getKey());
if (iEntryGroup == 1) {
assign(entry.getKey(), entry.getValue(), 2);
} else {
assign(entry.getKey(), entry.getValue(), 1);
}
}
The problem here is that each call to assign()will remove elements from group0, thus modifying its size, thus causing the following error:
这里的问题是每次调用assign()都会从 中删除元素group0，从而修改其大小，从而导致以下错误：
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
at java.util.HashMap$EntryIterator.next(HashMap.java:834)
at java.util.HashMap$EntryIterator.next(HashMap.java:832)
at liarliar$Bipartite.bipartition(liarliar.java:463)
at liarliar$Bipartite.readFile(liarliar.java:216)
at liarliar.main(liarliar.java:483)
So... how can I loop through the elements in group0while it's dynamically changing?
那么...如何在元素group0动态变化时循环遍历元素？
回答by Sean Patrick Floyd
Others have mentioned the correct solution without actually spelling it out. So here it is:
其他人提到了正确的解决方案，但实际上并未详细说明。所以这里是：
Iterator<Map.Entry<String, Integer>> iterator =
group0.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry<String, Integer> entry = iterator.next();
// determine where to assign 'entry'
iEntryGroup = hasBeenAccusedByGroup(entry.getKey());
if (iEntryGroup == 1) {
assign(entry.getKey(), entry.getValue(), 2);
} else {
assign(entry.getKey(), entry.getValue(), 1);
}
// I don't know under which conditions you want to remove the entry
// but here's how you do it
iterator.remove();
}
Also, if you want to safely change the map in your assign function, you need to pass in the iterator (of which you can only use the remove function and only once) or the entry to change the value.
此外，如果您想安全地更改分配函数中的映射，则需要传入迭代器（其中您只能使用 remove 函数并且只能使用一次）或更改值的条目。
回答by Bill K
As my answer here says:
正如我在这里的回答所说：
Iterating through a Collection, avoiding ConcurrentModificationException when removing in loop
迭代一个集合，在循环中删除时避免 ConcurrentModificationException
Use Iterator.remove()
使用 Iterator.remove()
回答by Kevin Williams
回答by CurtainDog
In your particular case I would not modify the structure of the HashMap but merely null the value you want to remove. Then if you end up visiting a null value just skip it.
在您的特定情况下，我不会修改 HashMap 的结构，而只会将您要删除的值设为 null。然后，如果您最终访问一个空值，只需跳过它。
In the general case I prefer to use a Stack for things like this because they're particularly easy to visualise and so I tend to have less issues with border conditions (just keeping popping 'till empty).
在一般情况下，我更喜欢使用 Stack 来处理这样的事情，因为它们特别容易可视化，所以我倾向于减少边界条件的问题（只是保持弹出直到空）。
回答by Affe
You need to use the actual iterator and its remove method if you want to modify the collection while looping over it. There isn't really any way to do it with the foreach construct.
如果要在循环遍历集合时修改集合，则需要使用实际的迭代器及其 remove 方法。使用 foreach 构造实际上没有任何方法可以做到这一点。
If you're trying to remove multiple entries in one iteration, you'll need to loop over something that's not backed by the map.
如果您尝试在一次迭代中删除多个条目，则需要循环遍历不受地图支持的内容。
Set<String> keys = new HashSet<String>(group0.keySet());
for (String key : keys) {
if (group0.containsKey(key)) {
Integer value = group0.get(key);
//your stuff
}
}
