Re: ConcurrentModificationException in single-threaded context

"Mike Schilling" <>
Wed, 23 Jul 2008 15:46:26 -0700
Eric Sosman wrote: wrote:

Got a weird CME when doing Map.puts or Map.gets on a private
which gets accessed by a SINGLE thread. I also never extract
iterators from the Map.. I only do put() and get().. yet sometimes
get ConcurrentModificationException. Toggling the Map between
Hashtable, HashMap or WeakHashMap implementations makes no
difference whatsoever. Black magic, or am I being dense?

    Meaning no insult, I suspect the latter.

    You say you "never extract iterators," but I bet you do without
realizing it. Note that the `for (Thing t : things)' loop is really
just shorthand for

for (Iterator<Thing> it = things.iterator(); it.hasNext(); ) {
    Thing t =;

so you may be using Iterators even if the string "Iterator" never
shows up in your source code.

    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
ConcurrentModificationException at the next hasNext() call.

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

In a single thread, the behavior should be deterministic.

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