ConcurrentModificationException的情况

ConcurrentModificationException
一个不该犯的低级错误,今天的代码突然抛了一个concurrentModificationException错误,
Iterator的一个基本概念没有掌握导致的这个错误,就是在Iterator的实现类
比如Hashtable里面的内部类
private class Enumerator<T> implements Enumeration<T>, Iterator<T>

会在next,或者remove的时候检查当前集合是否会在修改状态,如果是的话
就会抛出 ConcurrentModificationException,而他自己remove则是使用了同步的方法
而且同步了modCount;expectedModCount;

public T next() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
return nextElement();
}

public void remove() {
if (!iterator)
throw new UnsupportedOperationException();
if (lastReturned == null)
throw new IllegalStateException("Hashtable Enumerator");
if (modCount != expectedModCount)
throw new ConcurrentModificationException();

synchronized(Hashtable.this) {
Entry[] tab = Hashtable.this.table;
int index = (lastReturned.hash & 0x7FFFFFFF) % tab.length;

for (Entry<K,V> e = tab[index], prev = null; e != null;
prev = e, e = e.next) {
if (e == lastReturned) {
modCount++;
expectedModCount++;
if (prev == null)
tab[index] = e.next;
else
prev.next = e.next;
count--;
lastReturned = null;
return;
}
}
throw new ConcurrentModificationException();
}
}
}
而自己在next的同时,修改了这个集合,导致了这个错误的出现

在Map或者Collection的时候，不要用它们的API直接修改集合的内容，如果要修改可以用Iterator的remove()方法，例如：

public void setReparation( Reparation reparation ) {
for (Iterator it = this.reparations.iterator();it.hasNext();){//reparations为Collection
Reparation repa = (Reparation)it.next();
if (repa.getId() == reparation.getId()){
this.reparations.remove(repa);
this.reparations.add(reparation);
}
}
}

如上写会在运行期报ConcurrentModificationException，可以如下修改：

public void setReparation( Reparation reparation ) {
boolean flag = false;
for (Iterator it = this.reparations.iterator();it.hasNext();){//reparations为Collection
Reparation repa = (Reparation)it.next();
if (repa.getId() == reparation.getId()){
it.remove();
flag = true;
break;
}
}
if(flag){
this.reparations.add(reparation);
}
}