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[英]Java: `iterator.remove` throws IllegalStateException
[英]HashMap throws ConcurrentModification even if using iterator.remove() to remove node
操作HashMap
时出现ConcurrentModificationException
。 代码如下:
Iterator<Integer> iterator =cacheMap.keySet().iterator();
while(iterator.hasNext()) {
if(iterator.next() == target) {
iterator.remove();
}
}
我唯一使用的操作是这样删除和cacheMap.put(node)
。 这些方法都在主线程中调用,包括Activity
onCreate()
onPostExecute()
方法, AsyncTask
handleMessage(Message msg)
方法和主线程处理程序中的handleMessage(Message msg)
方法。
但是有时候,当我使用上述方法删除节点时,虽然它确实很少,但在iterator.next()
抛出了ConcurrentModificationException
。
我已经审查了HashMap中的相关方法。 就像这样:
@Override public Set<K> keySet() {
Set<K> ks = keySet;
return (ks != null) ? ks : (keySet = new KeySet());
}
private final class KeyIterator extends HashIterator
implements Iterator<K> {
public K next() { return nextEntry().key; }
}
private abstract class HashIterator {
int nextIndex;
HashMapEntry<K, V> nextEntry = entryForNullKey;
HashMapEntry<K, V> lastEntryReturned;
int expectedModCount = modCount;
HashIterator() {
if (nextEntry == null) {
HashMapEntry<K, V>[] tab = table;
HashMapEntry<K, V> next = null;
while (next == null && nextIndex < tab.length) {
next = tab[nextIndex++];
}
nextEntry = next;
}
}
public boolean hasNext() {
return nextEntry != null;
}
HashMapEntry<K, V> nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (nextEntry == null)
throw new NoSuchElementException();
HashMapEntry<K, V> entryToReturn = nextEntry;
HashMapEntry<K, V>[] tab = table;
HashMapEntry<K, V> next = entryToReturn.next;
while (next == null && nextIndex < tab.length) {
next = tab[nextIndex++];
}
nextEntry = next;
return lastEntryReturned = entryToReturn;
}
public void remove() {
if (lastEntryReturned == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
HashMap.this.remove(lastEntryReturned.key);
lastEntryReturned = null;
expectedModCount = modCount;
}
}
@Override public V remove(Object key) {
if (key == null) {
return removeNullKey();
}
int hash = secondaryHash(key);
HashMapEntry<K, V>[] tab = table;
int index = hash & (tab.length - 1);
for (HashMapEntry<K, V> e = tab[index], prev = null;
e != null; prev = e, e = e.next) {
if (e.hash == hash && key.equals(e.key)) {
if (prev == null) {
tab[index] = e.next;
} else {
prev.next = e.next;
}
modCount++;
size--;
postRemove(e);
return e.value;
}
}
return null;
}
private V removeNullKey() {
HashMapEntry<K, V> e = entryForNullKey;
if (e == null) {
return null;
}
entryForNullKey = null;
modCount++;
size--;
postRemove(e);
return e.value;
}
/**
* Subclass overrides this method to unlink entry.
*/
void postRemove(HashMapEntry<K, V> e) { }
调用nextEntry()
和remove
时, expectedModeCount
检查modCount
和nextEntry()
。 但是,如果调用iterator.remove()
删除节点,则两个整数之差似乎是不可能的。
只需使用cacheMap.remove(target)。
从文档中:
public V remove(Object key)从此映射中删除指定键的映射(如果存在)。
记住,HashMap只能为一个给定的键存储一个对象,因此不需要遍历所有值。
您无法迭代集合并从其中删除项目。
采用:
Set<Integer> set = new HashSet<Integer>();
Iterator<Integer> iterator =cacheMap.keySet().iterator();
while(iterator.hasNext()) {
Integer key = iterator.next();
if(key == target) {
set.add(key );
}
}
cacheMap.rmoveAll(set);
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