Re: ConcurrentModificationException in single-threaded context

From:

Eric Sosman <esosman@ieee-dot-org.invalid>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 23 Jul 2008 22:05:58 -0400

Message-ID:

<T7CdnYMmesoWfhrVnZ2dnUVZ_r7inZ2d@comcast.com>

Mike Schilling wrote:

Eric Sosman wrote:

[...]
From your description, I suspect `things' is either the keySet()
or entrySet() of the Map. If the "..." code executes put() on the
Map (or modifies the Map in any other way), the Iterator will
throw[*]
ConcurrentModificationException at the next hasNext() call.

[*] "Will very probably throw," really. See the Javadoc.

In a single thread, the behavior should be deterministic.

     If you're sure, file an RFE. ;-)

     To support it, exhibit concrete implementations that "work"
in all single-threaded situations, *including* those where forty-
two independent Iterators at forty-two independent positions are
simultaneously traversing the same HashMap.keySet() at the moment
when a new key/value pair is inserted and causes a re-hash ...
Repeat the exercise for all other collection classes ...

     A useful way to think about this is to imagine the Iterator
(or Enumeration) as using a "snapshot" of the collection to guide
its traversal. Change the collection, and it no longer matches
the snapshot, and the Iterator gets hopelessly lost.

--
Eric Sosman
esosman@ieee-dot-org.invalid