ConcurrentModificationException Within One Thread

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I at first thought it odd that a ConcurrentModificationException could be thrown w/in the context of a single Thread. But, what do you know, it can! And I seem to be getting better at writing code that does!

As a part of a recent hack, I wanted to remove a certain element from a list. As I was iterating over the list, I tried to find an object that met a certain criteria and then remove it, as follows:

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void removeStuff() {
   List<Recipient> list = service.populateList();
   for (Recipient r : list) {
      if (r.getProperty().equals(searchProperty)) {
         list.remove(r);
      }
   }
}

Unfortunately, this will throw the ConcurrentModificationException. From the said Exception’s JavaDoc:

“Note that this exception does not always indicate that an object has been concurrently modified by a different thread. If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception. For example, if a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will thow this exception.”

Perfect. That too-aptly describes my miserable attempts. So, it sounds like I just have to wait until iteration is complete to then attempt modification. So, a quick fix might look something like this:

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void removeStuff() {
   List<Recipient> list = service.populateList();
   Recipient toRemove = null;
   for (Recipient r : list) {
     if (r.getProperty().equals(searchProperty)) {
         toRemove = r;
      }
   }
   list.remove(toRemove);
}

Interestingly, and it was true in this case, the JavaDoc also states: “ConcurrentModificationException should be used only to detect bugs.”

Update

My friend, Steve, found a better way to do this inline w/o the exception that employs a manual iterator:

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Beaner beaner = null;
Iterator iter = list.iterator();
while (iter.hasNext()){
   beaner = (Beaner) iter.next();
   if (beaner.getType() == 3){
       iter.remove();
   }
}

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