Vector vs. Collections.synchronizedList(): Understanding the Differences

In Java, developers often need thread-safe data structures to handle concurrent access. Historically, Vector was the go-to synchronized collection, but modern Java provides Collections.synchronizedList(new ArrayList<>()) as an alternative. While both provide thread safety, they differ in how synchronization is applied and their overall performance.

This blog post will break down the key differences and help you decide which one to use.

Why Not Use Vector?

Vector is a legacy class that predates the Java Collections Framework. It provides thread safety by synchronizing all its methods, but this approach introduces unnecessary overhead in modern applications.

How Vector Works

import java.util.Vector;

Vector<String> vector = new Vector<>();
vector.add("Java");
vector.add("Python");
System.out.println(vector);

• Every method (e.g., add(), get(), remove()) is synchronized.
• Suitable for legacy code but generally not recommended for new applications due to performance concerns.

What About Collections.synchronizedList()?

Instead of using Vector, modern Java provides Collections.synchronizedList(), which synchronizes an ArrayList but offers more flexibility.

How It Works

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

List<String> list = Collections.synchronizedList(new ArrayList<>());
list.add("Java");
list.add("Python");
System.out.println(list);

• Only the wrapper methods are synchronized, meaning individual method calls like add(), get(), and remove() are thread-safe.
• However, iteration is NOT thread-safe by default. You need explicit synchronization when iterating.

Key Differences Between Vector and synchronizedList()

Feature	Vector	synchronizedList(new ArrayList<>())
Synchronization Level	Each method is synchronized individually.	Only the wrapper methods are synchronized.
Performance	Slower due to intrinsic synchronization.	Faster due to flexible synchronization.
Iteration Safety	Uses Enumeration or Iterator, but still needs external sync for iteration.	Requires explicit synchronization when iterating.
Preferred in Modern Java?	❌ No (outdated).	✅ Yes (better performance & flexibility).

The Important Caveat: Iteration in synchronizedList()

Even though synchronizedList() synchronizes individual method calls, iteration is NOT automatically thread-safe. If another thread modifies the list while iterating, it can cause a ConcurrentModificationException.

Unsafe Iteration (May Cause Errors)

for (String item : list) {  // 🚨 Not thread-safe
    System.out.println(item);
}

Safe Iteration (Explicit Synchronization Required)

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

When to Use What?

Use Vector If:

✅ You are working with legacy code that already uses Vector. ✅ You need built-in synchronization without extra effort. ❌ You want better performance.

Use Collections.synchronizedList(new ArrayList<>()) If:

✅ You need a thread-safe list with better performance than Vector. ✅ You want more flexibility over synchronization. ✅ You are handling explicit synchronization during iteration.

A Better Alternative: CopyOnWriteArrayList

For concurrent read-heavy scenarios, consider using CopyOnWriteArrayList, which avoids synchronization overhead by making copies during modifications.

import java.util.concurrent.CopyOnWriteArrayList;

CopyOnWriteArrayList<String> safeList = new CopyOnWriteArrayList<>();
safeList.add("Java");
safeList.add("Python");
System.out.println(safeList);

• No need for explicit synchronization during iteration.
• Ideal for read-heavy, write-infrequent scenarios.

Final Thoughts

• Vector is legacy and should generally be avoided unless maintaining old code.
• synchronizedList(new ArrayList<>()) is a better alternative but requires manual synchronization when iterating.
• CopyOnWriteArrayList is the best choice for read-heavy multi-threaded scenarios.