What Are the Key Differences Between CopyOnWriteArrayList and ArrayList in Java?

Java provides a range of built-in data structures, and two of the most commonly used list implementations are ArrayList and CopyOnWriteArrayList. While both are part of the Java Collections Framework, they serve different purposes, especially when it comes to thread safety and concurrent modifications. This article will explore the differences between CopyOnWriteArrayList and ArrayList in Java, explaining their functionality, use cases, performance characteristics, and providing code examples to demonstrate each.

1. Introduction to ArrayList and CopyOnWriteArrayList

Before delving into the differences, let’s define each of these classes:

• ArrayList: ArrayList is a part of the Java Collections Framework and implements the List interface. It is an ordered collection (or sequence) that allows duplicates and provides fast access to elements via an index. However, ArrayList is not thread-safe, meaning that it is not designed for concurrent modifications by multiple threads.
• CopyOnWriteArrayList: CopyOnWriteArrayList is part of the java.util.concurrent package and is a thread-safe variant of ArrayList. It is designed for situations where there are more reads than writes to the list. The “copy-on-write” strategy ensures thread safety by making a copy of the entire list when a modification occurs (such as an add, remove, or update operation).

2. Key Differences Between ArrayList and CopyOnWriteArrayList

The primary differences between ArrayList and CopyOnWriteArrayList can be classified into several categories:

2.1. Thread Safety

ArrayList:
ArrayList is not thread-safe, meaning that it is not safe for concurrent access by multiple threads. If multiple threads modify the list concurrently without proper synchronization, it can lead to data corruption, inconsistent behavior, or even application crashes.

• Scenario: If one thread is adding elements to an ArrayList while another is iterating over it, there is a risk of ConcurrentModificationException unless explicit synchronization is used.

CopyOnWriteArrayList:
In contrast, CopyOnWriteArrayList is thread-safe and designed for situations where reads significantly outnumber writes. It achieves thread safety by creating a new copy of the underlying array each time a modification is made (such as adding or removing an element).

• Scenario: Multiple threads can safely read from a CopyOnWriteArrayList without synchronization, as any modification will involve copying the entire list and providing each thread with its own consistent snapshot of the list.

2.2. Performance Considerations

ArrayList:

• Read Performance: ArrayList provides fast read operations (O(1) for get operations).
• Write Performance: Write operations (add, remove, set) are relatively fast in ArrayList, typically O(1) for appending elements to the end of the list or replacing an element at a specific index.
• Thread-Safety Impact: When ArrayList is used in multi-threaded environments, you need to manually synchronize access to avoid data inconsistency, which can hurt performance due to synchronization overhead.

CopyOnWriteArrayList:

• Read Performance: Since CopyOnWriteArrayList uses a snapshot of the underlying array, reads are extremely fast and lock-free. However, these reads involve copying the entire list whenever modifications occur, which can be costly in scenarios with frequent modifications.
• Write Performance: Write operations are much slower compared to ArrayList because each modification involves copying the entire underlying array. This makes it less efficient when the number of writes is high.

2.3. Use Cases

ArrayList:

• Use Case: ArrayList is ideal for scenarios where:
  • Thread safety is not a concern (i.e., single-threaded applications or explicit synchronization is used).
  • There are frequent modifications (adds and removes) to the list.
  • Quick access to elements by index is required.

CopyOnWriteArrayList:

• Use Case: CopyOnWriteArrayList is best used in scenarios where:
  • The list is read-heavy and modified infrequently.
  • The program requires thread-safe behavior without external synchronization mechanisms.
  • You need to maintain thread consistency during iteration, as it avoids ConcurrentModificationException.

2.4. Handling Concurrent Modifications

ArrayList:
When an ArrayList is modified while being iterated over (e.g., one thread modifies the list while another is iterating through it), a ConcurrentModificationException may be thrown. This is because the underlying array is changed during iteration, which violates the structure expected by the iterator.

CopyOnWriteArrayList:
CopyOnWriteArrayList handles concurrent modifications gracefully. When a thread modifies the list, it creates a copy of the internal array. This allows iterators to work with a consistent snapshot of the list, even while other threads are modifying the list. Therefore, ConcurrentModificationException is avoided in CopyOnWriteArrayList.

2.5. Iterator Behavior

ArrayList:

• The ArrayList iterator will throw a ConcurrentModificationException if the list is modified directly (i.e., outside the iterator’s own methods) while it is being iterated over.

ArrayList<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
Iterator<Integer> iterator = list.iterator();
list.add(3); // This will throw ConcurrentModificationException

CopyOnWriteArrayList:

• The iterator of CopyOnWriteArrayList does not throw ConcurrentModificationException even if the list is modified during iteration. This is because each modification results in a new copy of the internal array, so the iterator sees a consistent snapshot of the list.

CopyOnWriteArrayList<Integer> list = new CopyOnWriteArrayList<>();
list.add(1);
list.add(2);
Iterator<Integer> iterator = list.iterator();
list.add(3); // No exception, iterator sees a consistent snapshot

2.6. Memory Usage

ArrayList:
ArrayList uses a single underlying array to store elements, and it adjusts its size dynamically when elements are added. However, it doesn’t incur extra memory overhead for thread safety or snapshot copying.

CopyOnWriteArrayList:
CopyOnWriteArrayList uses more memory, as it maintains a separate copy of the entire array every time a modification occurs. This is especially costly when there are many modifications, as each write operation involves copying the entire list. This can result in high memory usage in write-heavy applications.

3. Code Examples

3.1. Example: ArrayList in a Single-threaded Context

import java.util.ArrayList;

public class ArrayListExample {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("Apple");
        list.add("Banana");
        list.add("Cherry");

        System.out.println("ArrayList: " + list);

        // Access by index
        System.out.println("Element at index 1: " + list.get(1));

        // Removing an element
        list.remove("Banana");
        System.out.println("After removal: " + list);
    }
}

3.2. Example: CopyOnWriteArrayList in a Multi-threaded Context

import java.util.concurrent.CopyOnWriteArrayList;

public class CopyOnWriteArrayListExample {
    public static void main(String[] args) {
        CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
        list.add("Apple");
        list.add("Banana");
        list.add("Cherry");

        // Simulating multi-threaded environment
        Runnable task = () -> {
            for (String fruit : list) {
                System.out.println(fruit);
                try { Thread.sleep(100); } catch (InterruptedException e) {}
            }
        };

        Thread t1 = new Thread(task);
        Thread t2 = new Thread(task);

        t1.start();
        t2.start();

        // Modifying the list while other threads are iterating
        list.add("Date");

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Final list: " + list);
    }
}

4. Summary of Differences

Feature	ArrayList	CopyOnWriteArrayList
Thread Safety	Not thread-safe	Thread-safe
Read Performance	Fast (O(1))	Fast (O(1))
Write Performance	Fast (O(1))	Slow due to copy-on-write
Iteration	May throw ConcurrentModificationException	Safe from ConcurrentModificationException
Use

Case | Single-threaded or synchronized use | Read-heavy, thread-safe environments | | Memory Usage | Efficient | Higher due to copying entire array on modification |

5. Conclusion

Choosing between ArrayList and CopyOnWriteArrayList depends on the specific use case. ArrayList is ideal for single-threaded environments or situations where thread safety is managed externally. On the other hand, CopyOnWriteArrayList is designed for scenarios with multiple threads, especially when the list is mostly read and infrequently modified. However, CopyOnWriteArrayList comes with performance trade-offs due to its copy-on-write mechanism, which can lead to significant overhead in write-heavy applications.

By understanding the differences in thread safety, performance, and use cases, you can make an informed decision about which list implementation to use in your Java applications.