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According to the Java API documentation [API 20062014] for the Iterator.remove() method:

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According to the Java API documentation [API 20062014] for ConcurrentModificationException:

It is not generally permissible for one thread to modify a Collection while another thread is iterating over it. In general, the results of the iteration are undefined under these circumstances. Some Iterator implementations (including those of all the general purpose collection implementations provided by the JRE) may choose to throw this exception if this behavior is detected. Iterators that do this are known as fail-fast iterators, as they fail quickly and cleanly, rather that risking arbitrary, non-deterministic behavior at an undetermined time in the future.
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Note that fail-fast behavior cannot be guaranteed because it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast operations throw ConcurrentModificationException on a best-effort basis. ConsequentlyTherefore, it would be wrong to write a program that depended on this exception for its correctness: ConcurrentModificationException should be used only to detect bugs.

Reliance on ConcurrentModificationException is inadequate to prevent undefined behavior resulting from modifying an underlying collection while simultaneously iterating over the collection. The fail-fast behavior may occur only after processing an arbitrary number of elements. In Java Concurrency in Practice [Goetz 2006a], Goetz and colleagues note:

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This noncompliant code example (based on Sun Developer Network SDN 2011 bug report 6687277) uses the ArrayList's remove() method to remove an element from an ArrayList while iterating over the ArrayList. The resulting behavior is unspecified.

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This approach must be implemented correctly to avoid starvation, deadlock, and scalability issues [Goetz 2006a].

Compliant Solution (Deep Copying)

This compliant solution creates a deep copy of the mutable widgetList before iterating over it.:

Code Block

bgColor	#ccccff

List<Widget> widgetList = new ArrayList<Widget>();

public void widgetOperation() {
  List<Widget> deepCopy = new ArrayList<Widget>();
  synchronized (widgetList) { // Client-side locking
    for (Object obj : widgetList) {
      deepCopy.add(obj.clone());
    }
  } 

  for (Widget w : deepCopy) {
    doSomething(w);
  }
}

Creating deep copies of the list prevents underlying changes in the original list from affecting the iteration in progress. "Since the clone is thread-confined, no other thread can modify it during iteration, eliminating the possibility of ConcurrentModificationException. (The collection still must be locked during the clone operation itself)" [Goetz 2006a]. However, this approach is often more expensive than other techniques. There is also a risk of operating on stale data, which may affect the correctness of the code.

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The CopyOnWriteArrayList data structure implements all mutating operations by making a fresh copy of the underlying array. It is fully thread-safe and is optimized for cases where in which traversal operations vastly outnumber mutations. Note that traversals of such lists always see the list in the state it had at the creation of the iterator (or enhanced for loop); subsequent modifications of the list are invisible to an ongoing traversal. Consequently, this solution is inappropriate when mutations of the list are frequent or when new values should be reflected in ongoing traversals.

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