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NoSuchElementException occurs when Iterating through Java ArrayList concurrently

I have a method similar to the one below:
public void addSubjectsToCategory() {
final List<Subject> subjectsList = new ArrayList<>(getSubjectList());
for (final Iterator<Subject> subjectIterator =
subjectsList.iterator(); subjectIterator.hasNext();) {
addToCategory(subjectIterator.next().getId());
}
}
When this runs concurrently for the same user (another instance), sometimes it throws NoSuchElementException. As per my understanding, sometimes subjectIterator.next() get executed when there are no elements in the list. This occurs when being accessed only. Will method synchronization solve this issue?
The stack trace is:
java.util.NoSuchElementException: null
at java.util.ArrayList$Itr.next(Unknown Source)
at org.cmos.student.subject.category.CategoryManager.addSubjectsToCategory(CategoryManager.java:221)
This stack trace fails at the addToCategory(subjectIterator.next().getId()); line.

The basic rule of iterators is that underlying collection must not be modified while the iterator is being used.
If you have a single thread, there seems to be nothing wrong with this code as long as getSubjectsList() does not return null OR addToCategory() or getId() have some strange side-effects that would modify the subjectsList. Note, however, that you could rewrite the for-loop somewhat nicer (for(Subject subject: subjectsList) ...).
Judging by your code, my best guess is that you have another thread which is modifying subjectsList somewhere else. If this is the case, using a SynchronizedList will probably not solve your problem. As far as I know, synchronization only applies to List methods such as add(), remove() etc., and does not lock a collection during iteration.
In this case, adding synchronized to the method will not help either, because the other thread is doing its nasty stuff elsewhere. If these assumptions are true, your easiest and safest way is to make a separate synchronization object (i.e. Object lock = new Object()) and then put synchronized (lock) { ... } around this for loop as well as any other place in your program that modifies the collection. This will prevent the other thread from doing any modifications while this thread is iterating, and vice versa.

subjectIterator.hasNext();) {
--- Imagine a thread switch occurs here, at this point, between the call to hasNext() and next() methods.
addToCategory(subjectIterator.next().getId());
What could happen is the following, assuming you are at the last element in the list:
thread A calls hasNext(), the result is true;
thread switch occurs to thread B;
thread B calls hasNext(), the result is also true;
thread B calls next() and gets the next element from the list; now the list is empty because it was the last one;
thread switch occurs back to thread A;
thread A is already inside the body of the for loop, because this is where it was interrupted, it already called hasNext earlier, which
was true;
so thread A calls next(), which fails now with an exception, because there are no more elements in the list.
So what you have to do in such situations, is to make the operations hasNext and next behave in an atomic way, without thread switches occurring in between.
A simple synchronization on the list solves, indeed, the problem:
public void addSubjectsToCategory() {
final ArrayBlockingQueue<Subject> subjectsList = new ArrayBlockingQueue(getSubjectList());
synchronized (subjectsList) {
for (final Iterator<Subject> subjectIterator =
subjectsList.iterator(); subjectIterator.hasNext();) {
addToCategory(subjectIterator.next().getId());
}
}
}
Note, however, that there may be performance implications with this approach. No other thread will be able to read or write from/to the same list until the iteration is over (but this is what you want). To solve this, you may want to move the synchronization inside the loop, just around hasNext and next. Or you may want to use more sophisticated synchronization mechanisms, such as read-write locks.

It sounds like another thread is calling the method and grabbing the last element while another thread is about to get the next. So when the other thread finishes and comes back to the paused thread there is nothing left. I suggest using an ArrayBlockingQueue instead of a list. This will block threads when one is already iterating.
public void addSubjectsToCategory() {
final ArrayBlockingQueue<Subject> subjectsList = new ArrayBlockingQueue(getSubjectList());
for (final Iterator<Subject> subjectIterator =
subjectsList.iterator(); subjectIterator.hasNext();) {
addToCategory(subjectIterator.next().getId());
}
}
There is a bit of a wrinkle that you may have to sort out. The ArrayBlockingQueue will block if it is empty or full and wait for a thread to either insert something or take something out, respectively, before it will unblock and allow other threads to access.

You can use Collections.synchronizedList(list) if all you need is a simple invocation Sycnchronization. But do note that the iterator that you use must be inside the Synchronized block.

As I get you are adding elements to a list which might be under reading process.
Imagine the list is empty and your other thread is reading it. These kinds of problems might lead into your problem. You could never be sure that an element is written to your list which you are trying to read , in this approach.

I was surprised not to see an answer involving the use of a CopyOnWriteArrayList or Guava's ImmutableList so I thought that I would add such an answer here.
Firstly, if your use case is such that you only have a few additions relative to many reads, consider using the CopyOnWriteArrayList to solve the concurrent list traversal problem. Method synchronization could solve your issue, but CopyOnWriteArrayList will likely have better performance if the number of concurrent accesses "vastly" exceeds the number of writes, as per that class's Javadoc.
Secondly, if your use case is such that you can add everything to your list upfront in a single-threaded manner and only then do you need iterate across it concurrently, then consider Guava's ImmutableList class. You accomplish this by first using a standard ArrayList or a LinkedList or a builder for your ImmutableList. Once your single-threaded data entry is complete, then you instantiate your ImmutableList using either ImmutableList.copyOf() or ImmutableList.build(). If your use case will allow for this write/read pattern, this will probably be your most performant option.
Hope that helps.

I would like to make a suggestion that would probably solve your problem, considering that this is a concurrency issue.
If making the method addSubjectsToCategory() synchronized solves your problem, then you have located where your concurrency issue is. It is important to locate where the problem occurs, otherwise the information you provided is useless to us, we can't help you.
IF using synchronized in your method solves your problem, then consider this answer as educational or as a more elegant solution. Otherwise, share the code where you implement your threading environment, so we can have a look.
public synchronized void addSubjectsToCategory(List subjectsList){
Iterator iterator = subjectsList.iterator();
while(iterator.hasNext())
addToCategory(iterator.next().getId());
}
or
//This semaphore should be used by all threads. Be careful not to create a
//different semaphore each time.
public static Semaphore mutex = new Semaphore(1);
public void addSubjectsToCategory(List subjectsList){
Iterator<Subject> iterator = subjectsList.iterator();
mutex.acquire();
while(iterator.hasNext())
addToCategory(iterator.next().getId());
mutex.release();
}
Synchronized is clean, tidy and elegant. You have a really small method and creating locks, imho is unnecessary.
Synchronized means that only 1 thread will be able to enter the method at a time. Which means, you should use it only if you want 1 thread active each time.
If you actually need parallel execution, then your problem is not thread-related, but has something to do with the rest of your code, which we can not see.

can i synchronize 2 different methods? java

I have some trouble with the use of the synchronized keyword in java. I do understand the part where threads get locks on methods or code blocks but I dont know how to use it in the following example.
I have got 2 different threads (Thread A and Thead B) and a Class1 that holds a list witch contains instances of Class2. The Class1.methodA() that gets called by threadA modifies the information in the list. The Class1.methodB() that gets called by threadB only uses the information in the list.
I concluded that problems that i am having in my program occur when Thread A is modifying the data in the list while Thread B is using it.
Should i create a synchronized method inside Class1 which than calls MethodA or MethodB (seems redundant to me). Or can the thread get a lock just on the specific instance of Class2 that is being modified?
I am sorry for any bad English.

Have both methods syncrhonize on the list:
methodA()
{
synchronized(list)
{
... use the list, no one else can touch it
}
... do other stuff.
}
same for methodB()
More info here.

There is a explanation about the threading here
Well, back to your question. You can actually just use the synchronize on your list and do the reading or adding in this synchronize block.
synchronized(yourList)
{
// do something
}
Otherwise I guess you can use the CopyOnWriteArrayList (API) from the java.util.concurrent package. But this is really costly, because it makes always a fresh copy of the underlying array.
CopyOnWriteArrayList<String> myArrayList = new CopyOnWriteArrayList<String>();
myArrayList .add("Stackoverflow");
Iterator<String> iterator = myArrayList .iterator();
while (iterator.hasNext())
System.out.println(iterator.next());
}
Another method would be to use the synchronizedList
List list = Collections.synchronizedList(new ArrayList());
synchronized(list) {
Iterator i = list.iterator();
while (i.hasNext())
foo(i.next());
}
But you still need the synchronized block in the end. The add and remove methods itself are atomic and thread safe, but iterating over the list is not.

Understanding collections concurrency and Collections.synchronized*

I learned yesterday that I've been incorrectly using collections with concurrency for many, many years.
Whenever I create a collection that needs to be accessed by more than one thread I wrap it in one of the Collections.synchronized* methods. Then, whenever mutating the collection I also wrap it in a synchronized block (I don't know why I was doing this, I must have thought I read it somewhere).
However, after reading the API more closely, it seems you need the synchronized block when iterating the collection. From the API docs (for Map):
It is imperative that the user manually synchronize on the returned map when iterating over any of its collection views:
And here's a small example:
List<O> list = Collections.synchronizedList(new ArrayList<O>());
...
synchronized(list) {
for(O o: list) { ... }
}
So, given this, I have two questions:
Why is this even necessary? The only explanation I can think of is they're using a default iterator instead of a managed thread-safe iterator, but they could have created a thread-safe iterator and fixed this mess, right?
More importantly, what is this accomplishing? By putting the iteration in a synchronized block you are preventing multiple threads from iterating at the same time. But another thread could mutate the list while iterating so how does the synchronized block help there? Wouldn't mutating the list somewhere else screw with the iteration whether it's synchronized or not? What am I missing?
Thanks for the help!

Why is this even necessary? The only explanation I can think of is
they're using a default iterator instead of a managed thread-safe
iterator, but they could have created a thread-safe iterator and fixed
this mess, right?
Iterating works with one element at a time. For the Iterator to be thread-safe, they'd need to make a copy of the collection. Failing that, any changes to the underlying Collection would affect how you iterate with unpredictable or undefined results.
More importantly, what is this accomplishing? By putting the iteration
in a synchronized block you are preventing multiple threads from
iterating at the same time. But another thread could mutate the list
while iterating so how does the synchronized block help there?
Wouldn't mutating the list somewhere else screw with the iteration
whether it's synchronized or not? What am I missing?
The methods of the object returned by synchronizedList(List) work by synchronizing on the instance. So no other thread could be adding/removing from the same List while you are inside a synchronized block on the List.

The basic case
All of the methods of the object returned by Collections.synchronizedList() are synchronized to the list object itself. Whenever a method is called from one thread, every other thread calling any method of it is blocked until the first call finishes.
So far so good.
Iterare necesse est
But that doesn't stop another thread from modifying the collection when you're between calls to next() on its Iterator. And if that happens, your code will fail with a ConcurrentModificationException. But if you do the iteration in a synchronized block too, and you synchronize on the same object (i.e. the list), this will stop other threads from calling any mutator methods on the list, they have to wait until your iterating thread releases the monitor for the list object. The key is that the mutator methods are synchronized to the same object as your iterator block, this is what's stopping them.
We're not out of the woods yet...
Note though that while the above guarantees basic integrity, it doesn't guarantee correct behaviour at all times. You might have other parts of your code that make assumptions which don't hold up in a multi-threaded environment:
List<Object> list = Collections.synchronizedList( ... );
...
if (!list.contains( "foo" )) {
// there's nothing stopping another thread from adding "foo" here itself, resulting in two copies existing in the list
list.add( "foo" );
}
...
synchronized( list ) { //this block guarantees that "foo" will only be added once
if (!list.contains( "foo" )) {
list.add( "foo" );
}
}
Thread-safe Iterator?
As for the question about a thread-safe iterator, there is indeed a list implementation with it, it's called CopyOnWriteArrayList. It is incredibly useful but as indicated in the API doc, it is limited to a handful of use cases only, specifically when your list is only modified very rarely but iterated over so frequently (and by so many threads) that synchronizing iterations would cause a serious bottle-neck. If you use it inappropriately, it can vastly degrade the performance of your application, as each and every modification of the list creates an entire new copy.

Synchronizing on the returned list is necessary, because internal operations synchronize on a mutex, and that mutex is this, i.e. the synchronized collection itself.
Here's some relevant code from Collections, constructors for SynchronizedCollection, the root of the synchronized collection hierarchy.
SynchronizedCollection(Collection<E> c) {
if (c==null)
throw new NullPointerException();
this.c = c;
mutex = this;
}
(There is another constructor that takes a mutex, used to initialize synchronized "view" collections from methods such as subList.)
If you synchronize on the synchronized list itself, then that does prevent another thread from mutating the list while you're iterating over it.
The imperative that you synchronize of the synchronized collection itself exists because if you synchronize on anything else, then what you have imagined could happen - another thread mutating the collection while you're iterating over it, because the objects locked are different.

Sotirios Delimanolis answered your second question "What is this accomplishing?" effectively. I wanted to amplify his answer to your first question:
Why is this even necessary? The only explanation I can think of is they're using a default iterator instead of a managed thread-safe iterator, but they could have created a thread-safe iterator and fixed this mess, right?
There are several ways to approach making a "thread-safe" iterator. As is typical with software systems, there are multiple possibilities, and they offer different tradeoffs in terms of performance (liveness) and consistency. Off the top of my head I see three possibilities.
1. Lockout + Fail-fast
This is what's suggested by the API docs. If you lock the synchronized wrapper object while iterating it (and the rest of the code in the system written correctly, so that mutation method calls also all go through the synchronized wrapper object), the iteration is guaranteed to see a consistent view of the contents of the collection. Each element will be traversed exactly once. The downside, of course, is that other threads are prevented from modifying or even reading the collection while it's being iterated.
A variation of this would use a reader-writer lock to allow reads but not writes during iteration. However, the iteration itself can mutate the collection, so this would spoil consistency for readers. You'd have to write your own wrapper to do this.
The fail-fast comes into play if the lock isn't taken around the iteration and somebody else modifies the collection, or if the lock is taken and somebody violates the locking policy. In this case if the iteration detects that the collection has been mutated out from under it, it throws ConcurrentModificationException.
2. Copy-on-write
This is the strategy employed by CopyOnWriteArrayList among others. An iterator on such a collection does not require locking, it will always show consistent results during iterator, and it will never throw ConcurrentModificationException. However, writes will always copy the entire array, which can be expensive. Perhaps more importantly, the notion of consistency is altered. The contents of the collection might have changed while you were iterating it -- more precisely, while you were iterating a snapshot of its state some time in the past -- so any decisions you might make now are potentially out of date.
3. Weakly Consistent
This strategy is employed by ConcurrentLinkedDeque and similar collections. The specification contains the definition of weakly consistent. This approach also doesn't require any locking, and iteration will never throw ConcurrentModificationException. But the consistency properties are extremely weak. For example, you might attempt to copy the contents of a ConcurrentLinkedDeque by iterating over it and adding each element encountered to a newly created List. But other threads might be modifying the deque while you're iterating it. In particular, if a thread removes an element "behind" where you've already iterated, and then adds an element "ahead" of where you're iterating, the iteration will probably observe both the removed element and the added element. The copy will thus have a "snapshot" that never actually existed at any point in time. Ya gotta admit that's a pretty weak notion of consistency.
The bottom line is that there's no simple notion of making an iterator thread safe that would "fix this mess". There are several different ways -- possibly more than I've explained here -- and they all involve differing tradeoffs. It's unlikely that any one policy will "do the right thing" in all circumstances for all programs.

Wrapped implementation of a map

What's the point of wrapping the map with Collections.synchronizedCollection(map), if then you have to synchronize the code while iterating?
Collection<Type> c = Collections.synchronizedCollection(myCollection);
synchronized(c) {
for (Type e : c)
foo(e); }
After having wrapped it, should not be thread safe?

What's the point of wrapping the map with Collections.synchronizedCollection(map), if then you have to synchronize the code while iterating?
To make individual operations thread-safe. (Personally I think it's a bad idea in general, but that's a different matter. It's not pointless, just limited in usefulness.)
After having wrapped it, should not be thread safe?
For any individual operation, yes. But iteration involves many steps - and while each of those individual steps will be synchronized, the collection can be modified between steps, invalidating the iterator. Don't forget that your loop is expanded to something like:
for (Iterator<Type> iterator = c.iterator(); iterator.hasNext(); ) {
Type e = iterator.next();
...
}
If you need iteration to be thread-safe, you should use one of the collections in java.util.concurrent... while noting the caveats about what is and isn't guaranteed if the collection is modified during iteration.

After wrapping it, each individual method is thread safe, but iteration involves calling methods repeatedly (iterator, then next and hasNext on the returned Iterator) and there's no synchronization between those methods. This is why you need to synchronize your iteration.
You also need to use a synchronized collection (rather than just synchronizing around your iteration code) because otherwise the methods that add or remove items would not synchronize and therefore could make modifications while you were iterating even if you used a synchronized block.

Adding to #jonskeet's #jule's answers, you should consider using ConcurrentHashMap (http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/ConcurrentHashMap.html) which does not require locking around iteration.

Iterator Concurrent Modifiction Exception

This code will throw Concurrent Modification Exception if the list is modified in doSomething(). Is it possible to avoid it by enclosing the code in some synchronized block?
List l = Collections.synchronizedList(new ArrayList());
// normal iteration -- can throw ConcurrentModificationException
// may require external synchronization
for (Iterator i=list.iterator(); i.hasNext(); ) {
doSomething(i.next());
}

if you are removing an item from the list, you can do it by calling iterator.remove() instead of list.remove(iterator.next())
if you are adding an item - well, create a copy of the iterated list and add it there
if the code snippet above is part of the same method, then you don't need a synchronized list or synchronized blocks - no other thread can access the local list.

You can modify a Collection while iterating over it if you do so through the Iterator interface. You can use Iterator.remove() to remove elements.

You cannot modify it while you are iterating over it. Synchronizing won't help here.
EDIT : I forgot iterator does have the remove method. So it is possible to remove.

I agree with others about Iterator and remove().
About synchronization, I wanted to add that synchronization is designed to control interactions between different threads.
It is typical for an object to have several methods synchronized, and that one would call another. So the language designers decided that the same thread would not be blocked by himself on a synchronized.
Also, thinking about it, it a thread is blocked waiting for himself, you have a magnificent starvation perspective! ;-)
So this answers one of your questions: it is not possible to avoid the problem by synchronizing your code.

Use CopyOnWriteArrayList instead of synchronized Array List

List l = Collections.synchronizedList(new ArrayList());
synchronized(l) {
// normal iteration -- can throw ConcurrentModificationException
// may require external synchronization
for (Iterator i=list.iterator(); i.hasNext(); ) {
doSomething(i.next());
}
}