How to fix a java.util.ConcurrentModificationException

Question

My code results in an error where I do not know how to fix it. I tried putting in print statements but It wont even make it that far. The error occurs

Here is the exact error

java.util.ConcurrentModificationException
java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.nextEntry(HashMap.java:894)
        at java.util.HashMap$KeyIterator.next(HashMap.java:928)
        at ca.on.oicr.pinery.lims.gsle.GsleClient.getOrders(GsleClient.java:720)

Line 720 is the second for loop

You're adding to orders inside a loop that's looping over the elements of orders, that's what causes the exception. Don't modify a collection you're looping over inside the loop. Likewise with samples. — Jesper
– Jesper, Commented Oct 29, 2013 at 14:15
You cannot modify (add or remove) the collection when you are looping over it. You are trying to add a order to orders. You cannot do that. — Abhijith Nagarajan
– Abhijith Nagarajan, Commented Oct 29, 2013 at 14:16

Kal · Accepted Answer · 2013-10-29 14:25:45Z

3

You can use a ListIterator if you want add or remove elements from a list while iterating over the elements. This is assuming that your orders is a List

So, your code would look something like this --

ListIterator<Order> it = orders.listIterator();

while ( it.hasNext() ) {
      Order ord = it.next();

      if ( ) // some condition
        it.remove(); // This wil remove the element that we just got using the next() method
      if ( ) // some other condition
        it.add(new Order()); // THis inserts the element immediately before the next call to next()
}

answered Oct 29, 2013 at 14:25

Kal

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2 Comments

user2811419 Over a year ago

in my case, would I be removing? and I am creating a new instance of Order everytime?

Kal Over a year ago

If the orders is a List, then you can use the ListIterator and use the add method

Guillaume · Accepted Answer · 2013-10-29 14:15:33Z

1

You're trying to manipulate the content of sample while iterating over its contents. To fix this kind of problems, use immutable collections, or pretend they are.

What you want to do is, while iterating over samples, build up another collection with the one you want, and modify this other collection instead of your original one.

answered Oct 29, 2013 at 14:15

Guillaume

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9 Comments

Jorge Campos Over a year ago

I think he has two problems samples and orders inside the samples loop he are changing the content of orders too.

Guillaume Over a year ago

Yes, I wish that default collections in Java were immutable, to prevent this kind of behaviour... (Like they are in Scala) - we need more functional programming concepts into our OO ways :)

user2811419 Over a year ago

to fix this would I just put the orders.add(order) outside the second forloop but within the first first loop? would that solve it?

user2811419 Over a year ago

or do I have to reconstruct this whole thing from scratch

Guillaume Over a year ago

You would have to rewrite it: You cannot modify a collection while iterating over it! The problem would still exist if you had only one collection (orders)

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