How to Avoid `ConcurrentModificationException` When Using Synchronized Methods?

Avoiding ConcurrentModificationException When Using Synchronized Methods

The ConcurrentModificationException error typically occurs due to modifications made to the collection an iterator is iterating over within the loop body. Despite the presence of a synchronized keyword in the method declaration, this exception can still be encountered.

In the provided code snippet:

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

The error is likely being thrown because an external thread is modifying the underlying collection while the iterator is iterating over it. This modification can include adding or removing elements.

The solution to this issue is to ensure that no other thread can modify the collection during iteration. This can be achieved by creating a copy of the collection before iterating over it. However, if this is not feasible, then the iterator must be obtained in a thread-safe manner. This can be done by using the Collections.synchronizedList() method or by creating a custom synchronization wrapper for the collection.

By taking these steps, you can ensure that the ConcurrentModificationException is not encountered and that the collection is iterated over safely.

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