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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:

Map<String, Integer> group0 = new HashMap<String, Integer>();

... and extract every element in group0. This is my approach:

// 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:

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 group0 while it's dynamically changing?

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6 に答える 6

7

他の人は、実際にそれを詳しく説明せずに正しい解決策に言及しました。だからここにあります：

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();
}

また、assign関数でマップを安全に変更する場合は、イテレータ（remove関数を1回だけ使用できます）またはエントリを渡して値を変更する必要があります。

于 2010-08-19T05:57:06.363 に答える
3

As my answer here says:

Iterating through a Collection, avoiding ConcurrentModificationException when removing in loop

Use Iterator.remove()

于 2010-08-19T00:38:18.133 に答える
1

You could use a ConcurrentHashMap

于 2010-08-19T01:39:09.893 に答える
1

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.

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).

于 2010-08-19T02:10:55.700 に答える
0

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.

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.

Set<String> keys = new HashSet<String>(group0.keySet());
for (String key : keys) {
  if (group0.containsKey(key)) {
    Integer value = group0.get(key);
    //your stuff 
  }
}
于 2010-08-19T00:02:55.280 に答える
0

In this case, how can assign modify group0? More details are needed. Typically you cannot modify a collection while iterating over it. You modify through the Iterator interface.

于 2010-08-19T00:32:25.497 に答える