What are the main differences between CopyOnWriteArrayList and ArrayList in Java?

The main differences between CopyOnWriteArrayList and ArrayList in Java are related to thread safety and performance:

• ArrayList is a non-thread-safe list. Concurrent access from multiple threads without external synchronization can lead to errors.

• CopyOnWriteArrayList is a thread-safe list that implements a copying strategy on write. When the list is modified (adding, removing), a new copy of the internal array is created, allowing safe reading from multiple threads without locks.

Implications:

• CopyOnWriteArrayList is well-suited for scenarios with frequent reads and infrequent modifications.
• Due to copying on write, modification operations are slower and require more memory.

Example of using CopyOnWriteArrayList:

CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
list.add("one");
list.add("two");
// Safe to read from multiple threads
for (String s : list) {
    System.out.println(s);
}