Posted on 2012-07-17 17:01
timelyxyz 閱讀(110)
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突然拋了一個concurrentModificationException錯誤,Iterator的一個基本概念沒有掌握導致的這個錯誤,就是在Iterator的實現類。比如Hashtable里面的內部類
private class Enumerator<T> implements Enumeration<T>, Iterator<T>
會在next,或者remove的時候檢查當前集合是否會在修改狀態,如果是的話，就會拋出 ConcurrentModificationException,而他自己remove則是使用了同步的方法，而且同步了modCount;expectedModCount;
1 public T next() {
2 if (modCount != expectedModCount)
3 throw new ConcurrentModificationException();
4 return nextElement();
5 }
6
7
8 public void remove() {
9 if (!iterator)
10 throw new UnsupportedOperationException();
11 if (lastReturned == null)
12 throw new IllegalStateException("Hashtable Enumerator");
13 if (modCount != expectedModCount)
14 throw new ConcurrentModificationException();
15
16 synchronized(Hashtable.this) {
17 Entry[] tab = Hashtable.this.table;
18 int index = (lastReturned.hash & 0x7FFFFFFF) % tab.length;
19
20 for (Entry<K,V> e = tab[index], prev = null; e != null; prev = e, e = e.next) {
22 if (e == lastReturned) {
23 modCount++;
24 expectedModCount++;
25 if (prev == null)
26 tab[index] = e.next;
27 else
28 prev.next = e.next;
29 count--;
30 lastReturned = null;
31 return;
32 }
33 }
34 throw new ConcurrentModificationException();
35 }
36 }
37 }
而自己在next的同時,修改了這個集合,導致了這個錯誤的出現
在Map或者Collection的時候，不要用它們的API直接修改集合的內容，如果要修改可以用Iterator的remove()方法，例如：
1 public void setReparation( Reparation reparation ) {
2 for (Iterator it = this.reparations.iterator();it.hasNext();){ //reparations為Collection
3 Reparation repa = (Reparation)it.next();
4 if (repa.getId() == reparation.getId()){
5 this.reparations.remove(repa);
6 this.reparations.add(reparation);
7 }
8 }
9 }
如上寫會在運行期報ConcurrentModificationException，可以如下修改：
1 public void setReparation( Reparation reparation ) {
2 boolean flag = false;
3 for (Iterator it = this.reparations.iterator();it.hasNext();){ //reparations為Collection
4 Reparation repa = (Reparation)it.next();
5 if (repa.getId() == reparation.getId()){
6 it.remove();
7 flag = true;
8 break;
9 }
10 }
11 if(flag){
12 this.reparations.add(reparation);
13 }
14 }
原文摘自 alreal