ConcurrentModificationException

簡單說就是，java的collection並沒有實作synchronized，

當在使用iterator時，若有其它的threed變更collection裡的原素，

將造成iterator裡的index與collection裡的元素不一致..

此時便會丟出ConcurrentModificationException…

Note that this implementation is not synchronized. If multiple threads access

a set concurrently, and at least one of the threads modifies the set, it must

be synchronized externally. This is typically complished by synchronizing on

some object that naturally encapsulates the set. If no such object exists,

the set should be “wrapped” using the Collections.synchronizedSet method.

This is best done at creation time, to prevent accidental unsynchronized

access to the HashSet instance:

Set s = Collections.synchronizedSet(new HashSet(…));

The iterators returned by this class’s iterator method are fail-fast: if the

set is modified at any time after the iterator is created, in any way except

through the iterator’s own remove method, the Iterator throws a

ConcurrentModificationException. Thus, in the face of concurrent

modification, the iterator fails quickly and cleanly, rather than risking

arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it

is, generally speaking, impossible to make any hard guarantees in the

presence of unsynchronized concurrent modification. Fail-fast iterators throw

ConcurrentModificationException on a best-effort basis. Therefore, it would

be wrong to write a program that depended on this exception for its

correctness: the fail-fast behavior of iterators should be used only to

detect bugs.

解決方法：

於宣告時，用Collections.synchronized包覆，

並在iterator迴圈外，用synchronized(){}包覆..

ex:

1. List list = Collections.synchronizedList(new ArrayList());
2. ...
3. synchronized(list) {
4. Iterator i = list.iterator(); // Must be in synchronized block
5. while (i.hasNext())
6. foo(i.next());
7. }