Posted on 2012-07-17 17:01
timelyxyz 閱讀(110)
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突然拋了一個(gè)concurrentModificationException錯(cuò)誤,Iterator的一個(gè)基本概念沒(méi)有掌握導(dǎo)致的這個(gè)錯(cuò)誤,就是在Iterator的實(shí)現(xiàn)類。比如Hashtable里面的內(nèi)部類
private class Enumerator<T> implements Enumeration<T>, Iterator<T>
會(huì)在next,或者remove的時(shí)候檢查當(dāng)前集合是否會(huì)在修改狀態(tài),如果是的話，就會(huì)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(shí),修改了這個(gè)集合,導(dǎo)致了這個(gè)錯(cuò)誤的出現(xiàn)
在Map或者Collection的時(shí)候，不要用它們的API直接修改集合的內(nèi)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(xiě)會(huì)在運(yùn)行期報(bào)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