MSC39-J. Do not modify the underlying collection when an iteration is in progress

According to the API documentation [[API 06]] for Iterator.remove() method:

The behavior of an iterator is unspecified if the underlying collection is modified while the iteration is in progress in any way other than by calling this method.

It is possible that in a multithreaded program, one thread iterates over a collection while another concurrently modifies the Collection. This can lead to unspecified behavior. Many implementations throw a ConcurrentModificationException when such a condition is detected.

According to the API documentation [[API 06]] 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.

Note that fail-fast behavior cannot be guaranteed as 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. Therefore, it would be wrong to write a program that depended on this exception for its correctness: ConcurrentModificationException should be used only to detect bugs.

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();
// ...

Risk Assessment

Modifying a Collection while iterating over it can lead to nondeterministic behavior unless the Iterator.remove() method is used.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[API 06]] Class ConcurrentModificationException
[[SDN 08]] Sun Bug database, Bug ID:6687277
[[Goetz 06]] 5.1.2. Iterators and Concurrentmodificationexception

MSC37-J. Make sensitive classes noncloneable      MSC39-J. Do not modify the underlying collection when an iteration is in progress      [|# 49. Miscellaneous (MSC)
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