Re: ConcurrentModificationException in single-threaded context

Eric Sosman <>
Wed, 23 Jul 2008 13:52:16 -0400
<1216835455.298060@news1nwk> wrote:

Got a weird CME when doing Map.puts or Map.gets on a private HashMap
which gets accessed by a SINGLE thread. I also never extract iterators
from the Map.. I only do put() and get().. yet sometimes I 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 throw[*]
ConcurrentModificationException at the next hasNext() call.

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


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