CopyOnWriteArrayList in Java

CopyOnWriteArrayList is a thread-safe, high-performance alternative to ArrayList, designed specifically for read-heavy and write-rare use cases. It belongs to the java.util.concurrent package.

The key idea is simple:

Every write operation creates a new copy of the internal array, ensuring that readers always see a consistent and safe snapshot without locks.

What is CopyOnWriteArrayList?

CopyOnWriteArrayList is a special list where:

All writes (add, remove, set) create a new array copy
Reads are extremely fast
No locking is required for reading
Iterators are fail-safe (never throw ConcurrentModificationException)
Perfect for concurrent read-mostly applications

Why “Copy On Write”? (Internal Working)

When you do:

list.add("A");

Internally, it performs:

Creates a new array
Copies old elements into the new array
Adds the new element
Replaces the old array reference

Safe in multi-threaded environments
Ultra-fast reads
Write operations are expensive (copying large arrays takes time)

Features of CopyOnWriteArrayList

Feature	Explanation
Thread-safe	Built-in concurrency without external synchronization
Copy on write mechanism	Every modification creates a new array
Fail-safe iterator	No ConcurrentModificationException
Read performance	Very fast (no locking)
Write performance	Slow (full array copy)
Null values	Allowed
Use case	Read-heavy, write-rare scenarios

When Should You Use CopyOnWriteArrayList?

Use it when:

Reads are very frequent
Writes are very rare
You need fast, consistent iteration
You want to avoid ConcurrentModificationException
Many threads are reading concurrently

Real-world use cases:

Caching data
Application configuration values
Observer / listener lists
Messaging subscribers
Read-only or rarely updated datasets

When NOT to Use It

Avoid CopyOnWriteArrayList when:

Frequent writes (copying on each write is slow)
Large lists (copy cost becomes huge)
Real-time or performance-critical write operations

Basic Example

import java.util.concurrent.CopyOnWriteArrayList;

public class Example {
    public static void main(String[] args) {
        CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();

        list.add("A");
        list.add("B");

        for (String s : list) {
            System.out.println(s);
        }
    }
}

Fail-Safe Iterator Example

CopyOnWriteArrayList iterates over a snapshot, so modifications during iteration don’t cause errors.

CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
list.add("One");
list.add("Two");

for (String s : list) {
    list.add("New"); // Safe — No ConcurrentModificationException
    System.out.println(s);
}

Output:

One
Two

("New" will not appear during this iteration because iterator uses a snapshot.)

Important Methods

Method	Description
`add(E e)`	Adds element (creates a new copy)
`remove(Object o)`	Removes element (new copy created)
`get(int index)`	Very fast read
`set(int index, E element)`	Updates element (copy on write)
`iterator()`	Returns fail-safe iterator

Difference Between CopyOnWriteArrayList vs ArrayList vs Vector

Feature	CopyOnWriteArrayList	ArrayList	Vector
Thread-safe?	Yes (internally uses copy-on-write)	No	Yes (every method synchronized)
Performance – Read	Fastest (no locking, snapshot-based)	Fast	Slow (due to locking)
Performance – Write	Slow (creates a new array on every write)	Fastest	Slow (global lock)
Iterator Type	Fail-safe (no ConcurrentModificationException)	Fail-fast	Fail-fast
Used for	Read-mostly, low-write operations	General purpose list	Legacy thread-safe code
Synchronization Overhead	Low for reads, high for writes	None	Very high (method-level sync)
Allows null?	Yes	Yes	Yes
Underlying Structure	Array	Array	Array
Package	`java.util.concurrent`	`java.util`	`java.util`
Best Use Case	Event listeners, caches, configuration lists	Everyday list usage	Legacy systems needing thread safety
Write During Iteration	Safe	Throws ConcurrentModificationException	Throws ConcurrentModificationException

Points to Remember

Thread-safe, optimized for concurrent reads
Writes are expensive because it copies the array
Iterators are fail-safe
Ideal for read-mostly applications
Part of java.util.concurrent

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