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The message is clear, do not rely on ConcurrentModificationException to stop any side effects resulting from modifying an underlying Collection while iterating over it. Notably, the enhanced for loop (for-each) internally uses an Iterator.

Noncompliant Code Example

This noncompliant code example (based on a bug report 6687277) removes an element from an ArrayList using the Collection's remove() method. This is done while iterating over the Collection. The resulting behavior is unspecified.

class BadIterate {
  public static void main(String[] args) {
  List<String> list = new ArrayList<String>();
  list.add("one");
  list.add("two");
        
  Iterator iter = list.iterator();
  while(iter.hasNext()) {
    String s = (String)iter.next();
    if(s.equals("one"))
      list.remove(s);
    }
  }    
}

Compliant Solution

The Iterator.remove() method removes from the underlying Collection the last element returned by the iterator. Its behavior is fully specified.

// ...
iter.remove();
// ...

Exceptions

EX1: The Iterator.remove() method can be used to modify the underlying collection when an iteration is in progress. This is also shown in the compliant solution.

Risk Assessment

Modifying a Collection while iterating over it can lead to nondeterministic behavior.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[API 06|AA. Java References#API 06]\] Class [ConcurrentModificationException|http://java.sun.com/j2se/1.5.0/docs/api/java/util/ConcurrentModificationException.html] 
\[[SDN 08|AA. Java References#SDN 08]\] [Sun Bug database, Bug ID:6687277|http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6687277]
\[[Goetz 06|AA. Java References#Goetz 06]\] 5.1.2. Iterators and Concurrentmodificationexception

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