Explain the working principle and features of CopyOnWriteArrayList in Java

CopyOnWriteArrayList is a thread-safe implementation of the List interface in Java that uses a copy-on-write strategy.

Working principle:

• Each modification operation (add, remove, update) creates a new copy of the internal array.
• Reading occurs without locks, directly from the current array.

Features:

• Ideal for scenarios with frequent reads and infrequent modifications.
• High read performance due to the absence of locks.
• Modifications are costly because of copying the entire array.
• Iterators do not throw ConcurrentModificationException and always work with an immutable copy of the data at the time of creation.

Example usage:

CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
list.add("A");
list.add("B");

for (String s : list) {
    System.out.println(s);
}

list.add("C"); // will create a new copy of the array

This list is convenient, for example, for storing event subscribers, where adding/removing occurs rarely, but notifications are frequent.