ConcurrentModificationException when adding inside a foreach loop in ArrayList

arraylist, for-loop, foreach, java

Solution

Java Collection classes are fail-fast which means that if the Collection will be changed while some thread is traversing over it using iterator, the `iterator.next()` will throw a `ConcurrentModificationException`.

This situation can come in case of multithreaded as well as single threaded environment. - www.javacodegeeks.com

You can't modify a `List` in a `for/each` loop, which is syntactic sugar around the `Iterator` as an implementation detail. You can only safely call `.remove()` when using the `Iterator` directly.

Note that Iterator.remove is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress. - Java Collections Tutorial

Calling `.add()` inside the `for/each` loop modifies the contents, and the `Iterator` that is used behind the scenes sees this and throws this exception.

A more subtle concern is the that the second way you list, the `.size()` is increasing every time you `.add()` so you will end up processing all the things you `.add()`, this could possibly cause an endless loop depending on what the input data is. I am not sure if this is what you desire.

Solution

I would create another `ArrayList` and `.add()` to it all the new things, and then after the loop, use `.addAll()` on the original `ArrayList` to combine the two lists together. This will make things explicit in what you are trying to do, that is unless your intention is process all the newly added things as you add them.

2014 Solution:

Always use `Immutable` collections classes and build new `Immutable` collection classes instead of trying to modify a single shared one. This is basically what my 2012 answer says but I wanted to make it more explicit.

Guava supports this very well, use `ImmutableList.copyOf()` to pass around data.

Use `Iterables.filter()` to filter out stuff into a new `ImmutableList`, no shared mutable state, means no concurrency problems!

Problem

Original source

Related problems

• ConcurrentModificationException despite using synchronized
• Concurrent Modification Exception : adding to an ArrayList
• Concurrent Modification Exception : adding to an ArrayList