Re: ConcurrentModificationException in single-threaded context

From:
"Mike Schilling" <mscottschilling@hotmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 24 Jul 2008 00:12:32 -0700
Message-ID:
<AXVhk.33884$ZE5.31106@nlpi061.nbdc.sbc.com>
Eric Sosman wrote:

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 ...


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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