ArrayList is not synchronized by default because synchronization comes at a cost of performance, and not all use cases require thread-safety.
Synchronization is the process of coordinating access to shared resources, like data structures, by multiple threads in a concurrent system. When a data structure is synchronized, the threads must acquire a lock before accessing the shared resource to prevent race conditions and ensure consistency of the data.
However, synchronizing an ArrayList by default can impact performance because it adds overhead to every method call, even if the ArrayList is being used by a single thread. In many use cases, the performance cost of synchronization is unnecessary because the ArrayList is only accessed by a single thread, or because the application is designed to manage concurrency in a different way.
If you do need to use an ArrayList in a multi-threaded environment, you can use the Collections.synchronizedList method to create a synchronized wrapper around an existing ArrayList instance, which provides thread-safety. Alternatively, you can use a different data structure that is already designed for thread-safety, such as CopyOnWriteArrayList.
