CopyOnWriteArrayList

Overview

• CopyOnWriteArrayList is a thread-safe variant of ArrayList
• Part of java.util.concurrent
• Uses copy-on-write strategy

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Core Idea

👉 On every write operation:

• A new copy of the array is created
• Modification happens on the new copy

👉 Reads:

• Use old snapshot
• No locking needed

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Key Characteristics

Feature	Description
Thread Safety	Yes
Read Performance	Very fast (no lock)
Write Performance	Slow (copy happens)
Iterator Type	Fail-safe (snapshot-based)
Duplicates	Allowed
Null Values	Allowed

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Internal Working

Original Array → Copy → Modify → Replace reference

Example:

[1,2,3] → copy → [1,2,3] → add(4) → [1,2,3,4]

Important Methods

Method Signature	Description	Behavior / Edge Case
boolean add(E e)	Adds element	Creates new copy
E get(int index)	Retrieves element	No locking
E set(int index, E element)	Updates value	Copy happens
boolean remove(Object o)	Removes element	Copy happens
Iterator<E> iterator()	Returns iterator	Snapshot-based

Key Rule

Iterator works on a snapshot of the array at the time of creation

Example 1: Modification During Iteration

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

    list.add(1);
    list.add(2);
    list.add(3);

    for (Integer i : list) {
        if (i == 2) {
            list.add(4); // ✅ allowed
        }
        System.out.println(i);
    }
}

Output:

1
2
3

👉 Notice:

4 is NOT printed
Because iterator is using old snapshot

Example 2: After Iteration

System.out.println(list);

Output:

[1, 2, 3, 4]

👉 Modification happened, but not visible during iteration

Example 3: No ConcurrentModificationException

for (Integer i : list) {
list.remove(i); // ✅ allowed
}

👉 No exception thrown

Example 4: Iterator Remove (IMPORTANT ⚠️)

Iterator<Integer> it = list.iterator();

while (it.hasNext()) {
it.next();
it.remove(); // ❌ UnsupportedOperationException
}

👉 Why?

Iterator is read-only snapshot

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Operation	Allowed	Visible in Current Iteration
Add	✅	❌
Remove	✅	❌
Update	✅	❌
Iterator remove()	❌	N/A

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Performance

Operation	Complexity
Read	O(1)
Write	O(n) (copy array)

When to Use

• Read-heavy applications
• Rare writes
• Multi-threaded environments

Examples:

• Caching
• Event listeners
• Configuration data

When NOT to Use

• Frequent writes ❌
• Large data sets ❌
• Memory-sensitive systems ❌

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CopyOnWriteArrayList vs ArrayList

Feature	ArrayList	CopyOnWriteArrayList
Thread Safe	❌	✅
Iterator	Fail-fast	Fail-safe
Writes	Fast	Slow
Reads	Fast	Very fast

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