CopyOnWriteArraySet vs HashSet in Java

1. Introduction

In Java, sets are collections that contain no duplicate elements. HashSet is one of the most widely used Set implementations and is known for its performance. Conversely, CopyOnWriteArraySet is a thread-safe variant of Set that achieves its thread safety by making a fresh copy of the underlying array with every mutation.

2. Key Points

1. HashSet is implemented as a hash table and offers constant-time performance for basic operations, assuming the hash function disperses elements properly.

2. CopyOnWriteArraySet is backed by a CopyOnWriteArrayList and provides thread safety by copying its entire contents on every write operation.

3. HashSet is not thread-safe and requires external synchronization to be used in concurrent scenarios.

4. CopyOnWriteArraySet does not require external synchronization and is suitable for situations with frequent reads and infrequent writes.

3. Differences: CopyOnWriteArraySet vs HashSet in Java

HashSet CopyOnWriteArraySet
Implements the Set interface and is backed by a CopyOnWriteArrayList. Implements the Set interface and is backed by a HashMap.
Designed for environments where read operations vastly outnumber write operations. Designed for general-purpose use with a good balance of read and write operations.
Iterators are fail-fast and can throw ConcurrentModificationException. Iterators do not throw ConcurrentModificationException since they work on a snapshot of the array.
It offers thread safety without additional synchronization when iterating, making it well-suited for high-concurrency scenarios. It is not thread-safe; external synchronization is needed for concurrent modification by multiple threads.
Does not allow null elements. Allows one null element.
Typically, it has a higher memory footprint and lower write performance than HashSet. More memory-efficient for many elements and offers better performance for write operations.

4. Example

// Import the necessary classes
import java.util.Set;
import java.util.HashSet;
import java.util.concurrent.CopyOnWriteArraySet;

public class SetComparison {
    public static void main(String[] args) {
        // Step 1: Create a HashSet
        Set<String> hashSet = new HashSet<>();
        // Step 2: Create a CopyOnWriteArraySet
        Set<String> copyOnWriteArraySet = new CopyOnWriteArraySet<>();

        // Step 3: Add elements to the HashSet

        // Step 4: Add elements to the CopyOnWriteArraySet

        // Step 5: Print both sets
        System.out.println("HashSet: " + hashSet);
        System.out.println("CopyOnWriteArraySet: " + copyOnWriteArraySet);


HashSet: [Python, JavaScript, Java]
CopyOnWriteArraySet: [Java, Python, JavaScript]


1. Two sets, HashSet and CopyOnWriteArraySet, are created to demonstrate their behavior with the same elements.

2. Elements are added to the HashSet; the order of elements in the output can differ due to the hash-based nature.

3. Elements are added to the CopyOnWriteArraySet; the iteration order is predictable and follows the insertion order.

4. Both sets are printed, showing they contain the same elements but may have different iteration orders.

5. When to use?

- Use HashSet when you need a general-purpose Set implementation, have no concurrency concerns, and prioritize performance.

- Opt for CopyOnWriteArraySet when thread safety is a priority, particularly in contexts where set mutations are infrequent and iteration over the set is a common operation.