CopyOnWriteArraySet in Java

CopyOnWriteArraySet is a thread-safe Set implementation in Java that is based on CopyOnWriteArrayList.

It is part of the java.util.concurrent package and is useful in multi-threaded scenarios where reads are more frequent than writes.

What is CopyOnWriteArraySet?

A Set that does not allow duplicates
Internally backed by CopyOnWriteArrayList
Thread-safe without using explicit synchronization
Iterators are fail-safe (do not throw ConcurrentModificationException)
Ideal for read-heavy collections

Package

import java.util.concurrent.CopyOnWriteArraySet;

Features of CopyOnWriteArraySet

Feature	Description
Thread-safe	Supports concurrent access without external locks
No duplicates	Set semantics – duplicates are not allowed
Fail-safe iterator	Iterators work on a snapshot of the array
Backed by	CopyOnWriteArrayList
Performance	Writes are costly (copying array), reads are very fast
Null elements	Allows one null element

Where CopyOnWriteArraySet Fits in Collection Hierarchy

Iterable
   └── Collection
         └── Set
               └── CopyOnWriteArraySet

Implements:

Set<E>
Serializable
Cloneable

How CopyOnWriteArraySet Works Internally

Every mutative operation (add, remove, etc.) creates a new copy of the internal array.
Iterators work on a snapshot of the array at the time of iterator creation → no ConcurrentModificationException.
Reading operations (contains, size, iteration) are fast and non-blocking.

Constructors

Default Constructor

CopyOnWriteArraySet<String> set = new CopyOnWriteArraySet<>();

Constructor with Collection

List<String> list = Arrays.asList("A", "B", "C");
CopyOnWriteArraySet<String> set = new CopyOnWriteArraySet<>(list);

Common Methods

Method	Description
`add(E e)`	Adds element if not present
`remove(Object o)`	Removes element if present
`contains(Object o)`	Checks if element exists
`size()`	Returns number of elements
`isEmpty()`	Checks if set is empty
`iterator()`	Returns fail-safe iterator
`clear()`	Removes all elements
`addAll(Collection c)`	Adds all elements from another collection

Example

import java.util.concurrent.CopyOnWriteArraySet;

public class Main {
    public static void main(String[] args) {
        CopyOnWriteArraySet<String> set = new CopyOnWriteArraySet<>();

        set.add("Apple");
        set.add("Banana");
        set.add("Cherry");
        set.add("Apple"); // Duplicate ignored

        System.out.println("Set: " + set);

        set.remove("Banana");

        System.out.println("After removal: " + set);

        // Iteration
        for (String s : set) {
            System.out.println(s);
        }
    }
}

Output:

Set: [Apple, Banana, Cherry]
After removal: [Apple, Cherry]
Apple
Cherry

When to Use CopyOnWriteArraySet

When reads are frequent and writes are rare
For event listener lists
For cache of read-mostly data
When thread-safe iteration without locks is needed

Advantages

Thread-safe without explicit locks
Fail-safe iterators
Fast read operations
No duplicates allowed

Disadvantages

Expensive write operations (array copy)
Not suitable for write-heavy workloads
Consumes more memory for large sets

Time Complexity

Operation	Complexity
`add()`	O(n) (due to array copy)
`remove()`	O(n)
`contains()`	O(n)
`size()`	O(1)
Iteration	O(n)

CopyOnWriteArraySet vs HashSet in Java

Feature	CopyOnWriteArraySet	HashSet
Thread-safety	Thread-safe, internally synchronized (safe for concurrent access)	Not thread-safe; external synchronization needed for multi-threading
Underlying Structure	Backed by `CopyOnWriteArrayList`	Backed by a `HashMap`
Performance	Reads are very fast (no locks), writes are expensive (entire array copied on modification)	Fast for add, remove, and contains (hash-based)
Iteration	Iterators reflect a snapshot; no `ConcurrentModificationException`	Fail-fast iterators; throw `ConcurrentModificationException` if modified during iteration
Use-case	Ideal for mostly-read, rarely-written scenarios (event listeners, caching)	Ideal for general-purpose sets with frequent add/remove operations
Allows null	Yes, allows one null element	Yes, allows one null element
Duplicates	No duplicates allowed	No duplicates allowed
Memory	Higher memory usage for writes (copies array on every modification)	Memory-efficient for most scenarios

Difference Between CopyOnWriteArraySet & CopyOnWriteArrayList

Feature	CopyOnWriteArraySet	CopyOnWriteArrayList
Interface Implemented	Implements Set interface	Implements List interface
Duplicates Allowed	No duplicates allowed (Set property)	Duplicates allowed (List property)
Ordering	No guaranteed order (insertion order maintained due to underlying list)	Maintains insertion order
Thread-safety	Thread-safe; all mutative operations create a new copy	Thread-safe; all mutative operations create a new copy
Underlying Structure	Backed by CopyOnWriteArrayList internally	Backed by array
Iteration Behavior	Iterators reflect a snapshot, safe in concurrent access	Iterators reflect a snapshot, safe in concurrent access
Best Use-case	Mostly-read sets; where uniqueness is required	Mostly-read lists; duplicates and order matter
Performance	Writes are expensive (array copied on every add/remove)	Writes are expensive (array copied on every add/remove)
Allows null	Yes, allows one null	Yes, allows multiple nulls

Points to Remember

CopyOnWriteArraySet = Thread-safe Set based on CopyOnWriteArrayList, ideal for read-heavy, low-write multi-threaded applications.

Allows fail-safe iteration
No duplicates
Writes are costly, reads are fast