What will happen if you use a regular ArrayList instead of a CopyOnWriteArrayList?

Using a regular ArrayList instead of CopyOnWriteArrayList in a multithreaded environment where reads are frequent and writes are rare can lead to thread safety issues. ArrayList is not synchronized, and concurrent access from multiple threads can cause:

• Incorrect data due to race conditions
• ConcurrentModificationException exceptions
• Corruption of the internal list structure

CopyOnWriteArrayList solves this problem by creating a copy of the internal array upon each modification, allowing safe reading without locks.

If you use ArrayList in such a situation, you need to ensure synchronization yourself, for example, using Collections.synchronizedList or external locks.

Example of the problem:

List<String> list = new ArrayList<>();
// Thread 1 reads list
// Thread 2 adds an element to list
// Without synchronization, errors or inconsistent states are possible

Therefore, for multithreaded access, CopyOnWriteArrayList or other thread-safe collections are preferable.