Re: ConcurrentModificationException in single-threaded context

"Mike Schilling" <>
Thu, 24 Jul 2008 00:12:32 -0700
Eric Sosman wrote:

Mike Schilling wrote:

Eric Sosman wrote:

    From your description, I suspect `things' is either the
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.

    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 ...

It's very simple: all 42 will throw CMEs when next accessed. The
implementation is quite simple. The underlying map has a "number of
modifications" counter that it increments whenever it's modified, e.g.
when a new key is added. Each iterator makes a copy of that number
when it's created. Whenever the iterator is accessed and its
modification count doesn't match the map's, it throws a CME. In a
single-threaded environment, this works perfectly. The weasel-wording
in the Javadoc about "will probably throw" is to cover multi-threaded
cases; since the accesses to the two modification counts are not
synchronized, it's possible that a change to them made in one thread
won't yet be visible in another.

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