In what situations is the CopyOnWriteArrayList suitable?

I am learning about the CopyOnWriteArrayList class.

• What is the purpose of copying a new array?
• Is it for other threads to read the array?

So if a system has high concurrency and most of the threads' actions are reading not writing, it is better to use CopyOnWriteArrayList.

As stated on this link:

CopyOnWriteArrayList is a concurrent Collection class introduced in Java 5 Concurrency API along with its popular cousin ConcurrentHashMap in Java.

CopyOnWriteArrayList implements List interface like ArrayList, Vector and LinkedList but its a thread-safe collection and it achieves its thread-safety in a slightly different way than Vector or other thread-safe collection class.

> As name suggest CopyOnWriteArrayList creates copy of underlying > ArrayList with every mutation operation e.g. add or set. Normally > CopyOnWriteArrayList is very expensive because it involves costly > Array copy with every write operation but its very efficient if you > have a List where Iteration outnumber mutation e.g. you mostly need to > iterate the ArrayList and don't modify it too often.

> Iterator of CopyOnWriteArrayList is fail-safe and doesn't throw > ConcurrentModificationException even if underlying > CopyOnWriteArrayList is modified once Iteration begins because > Iterator is operating on separate copy of ArrayList. Consequently all > the updates made on CopyOnWriteArrayList is not available to Iterator.

To get the most updated version do a new read like list.iterator();

That being said, updating this collection alot will kill performance. If you tried to sort a CopyOnWriteArrayList you'll see the list throws an UnsupportedOperationException (the sort invokes set on the collection N times). You should only use this read when you are doing upwards of 90+% reads.

> What is the purpose of coping a new array?

Copying the underlying array guarantees that any iteration of the data structure is safe as the iteration is occurring over an essentially immutable "snapshot" of the data.

> Is it for other threads to read the array?

Sort of. More specifically, it is for every thread to be able to safely iterate the array without fear of a ConcurrentModificationException or other unknown/undefined behavior.

> So if a system is high concurrency and most of the threads' actions are reading not writing, it is better to use CopyOnWriteArrayList. Am I right?

No. Only if most of the threads' actions are iterations over the list. If most of the activities are random access based reads, a ReadWriteLock might be better.

From the javadoc of CopyOnWriteArrayList

> This is ordinarily too costly, but may be more efficient than alternatives when traversal operations vastly outnumber mutations, and is useful when you cannot or don't want to synchronize traversals, yet need to preclude interference among concurrent threads.