HashMap中的并发修改异常

Question

I'm writing this program in Java and I'm getting a java.util.ConcurrentModificationException. The code excerpt is given below, please let me know if more code is required.

for (String eachChar : charsDict.keySet()) {
    if (charsDict.get(eachChar) < 2) {
        charsDict.remove(eachChar);
    }
}

charsDict is defined as

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

Please help me :)

Answer 1

You're not allowed to remove elements from the map while using its iterator.

A typical solution to overcome this:

List<String> toBeDeleted = new ArrayList<String>();
for (String eachChar : charsDict.keySet()) {
    if (charsDict.get(eachChar) < 2) {
        toBeDeleted.add(eachChar);
    }
}

for (String eachChar : toBeDeleted) {
    charsDict.remove(eachChar);
}

Answer 2

You need to use the remove method of the iterator:

for (Iterator<String> it = charsDict.keySet().iterator(); it.hasNext();) {
    String eachChar = it.next();
    if (charsDict.get(eachChar) < 2) {
        it.remove();
    }
}

Also note that since you need to access the key AND the value, it would be more efficient to use entrySet instead :

for (Iterator<Map.Entry<String, Integer>> it = charsDict.entrySet().iterator(); it.hasNext();) {
    Map.Entry<String, Integer> e = it.next();
    String eachChar = e.getKey();
    int value = e.getValue();
    if (value < 2) {
        it.remove();
    }
}

And finally it appears that the key is actually not used , so the loop becomes:

for (Iterator<Integer> it = charsDict.values().iterator(); it.hasNext();) {
    if (it.next() < 2) {
        it.remove();
    }
}

See also this related post .

Answer 3

如果在线程之间进行sahred，则使用ConcurrentHashMap可能是更好的选择... Iterator不是线程安全的，你应该创建一个新的迭代器，它在线程之间使用相同的东西。