Why Does a `ConcurrentModificationException` Occur Despite Using `synchronized` in Java?

ConcurrentModificationException despite Synchronization

In this code snippet:

public synchronized X getAnotherX() {
  if (iterator.hasNext()) {
    X b = iterator.next();
    String name = b.getInputFileName();
    ...
    return b;
  } else {
    return null;
  }
}

a ConcurrentModificationException is thrown despite the use of the synchronized keyword. This exception usually arises not from multi-threading issues, but rather from modifications to the underlying collection during iteration.

In this specific example, the error occurs when iterating through the collection with iterator.hasNext() and attempting to access an element with iterator.next(). If the underlying collection is modified while this iteration is in progress, such as adding or removing elements, it can cause the ConcurrentModificationException.

The solution lies in avoiding collection modifications during iteration. This can be achieved by creating a copy of the collection or by explicitly checking and handling these modifications within the iteration loop. Additionally, if the collection type supports a ListIterator, this interface provides additional methods for managing insertions and removals during iteration.

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