What can you tell about CopyOnWrite collections in Java?

CopyOnWriteArrayList and CopyOnWriteArraySet are implementations of the List and Set interfaces respectively, included in the java.util.concurrent package.

Their key feature is that mutative operations (add, remove, set) create a new copy of the internal array, which the collection object then references. Reading, however, occurs using the current array.

Advantages:

• Thread safety: Do not require explicit external synchronization for read and write operations.
• Read performance: Read operations are very fast as they do not block.
• Iterators: Iterators of these collections work with a snapshot of data at the moment of their creation. They do not throw ConcurrentModificationException when the collection is modified by another thread.

Disadvantages:

• Overhead of write operations: Each write operation (add, remove, set) creates a new copy of the array, which can be costly in terms of time and memory for large collections or frequent modifications.
• Data freshness: Iterators may not see the latest changes made after their creation.
• Memory consumption: Copying the array can lead to increased memory usage.

Typical use cases:

• When read operations significantly outnumber write operations.
• When high-performance reading without locks is required.
• When iterators need to be resilient to changes made by other threads.
• In scenarios where some delay between writing and seeing the change during reading is acceptable.

Example of using CopyOnWriteArrayList:

import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;

// Creating a thread-safe list
List<String> threadSafeList = new CopyOnWriteArrayList<>();

// Adding elements (creates a new array copy)
threadSafeList.add("Element 1");
threadSafeList.add("Element 2");

// Reading elements (uses the current array)
for (String element : threadSafeList) {
    System.out.println(element);
}

// Modifying from another thread during iteration
// The iterator will continue to work with the data at the time of its creation

Compared to other thread-safe collections, such as synchronized wrappers (Collections.synchronizedList) or collections from the java.util.concurrent package (e.g., ConcurrentHashMap), CopyOnWrite collections offer a specific trade-off between read and write performance and thread safety. They are ideal for scenarios where "read many, write little".