Synchronized Collections vs Concurrent Collections in Java – Which One to Use for Thread Safety and Performance?

By Ashwani Kumar Last updated: 14 Aug 2025

When building multithreaded applications in Java, one of the most critical decisions is how to manage shared data safely. Should you use synchronized collections or prefer concurrent collections like ConcurrentHashMap or CopyOnWriteArrayList?

Both have their use cases, trade-offs, and internal implementations. Choosing the wrong one can lead to performance bottlenecks or even data corruption. In this guide, we’ll explore the pros, cons, and best practices to help you pick the right tool.

📌 Core Definitions and Purpose

Synchronized Collections

Synchronized wrappers provided by Collections.synchronizedXXX() (e.g., synchronizedList, synchronizedMap)
Legacy approach to thread safety
Synchronizes every access to the collection using a single lock

Concurrent Collections

Introduced in Java 5 with java.util.concurrent package
Designed for high concurrency and fine-grained locking
Includes ConcurrentHashMap, CopyOnWriteArrayList, ConcurrentLinkedQueue, etc.

📦 Java Syntax and Usage

Synchronized List Example

import java.util.*;

List<String> syncList = Collections.synchronizedList(new ArrayList<>());

synchronized (syncList) {
    syncList.add("Apple");
    syncList.add("Banana");
}

⚠️ You must manually synchronize iteration over synchronized collections.

ConcurrentHashMap Example

import java.util.concurrent.*;

ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("A", 1);
map.put("B", 2);

map.forEach((k, v) -> System.out.println(k + ": " + v));

✅ No need to manually synchronize iteration.

⚙️ Internal Working and Memory Model

Synchronized Collections

Uses a single intrinsic lock (object monitor)
All access (read/write/iterate) is mutually exclusive
Causes high contention under load

Concurrent Collections

Use segment locks, non-blocking algorithms, or copy-on-write strategies
ConcurrentHashMap uses bucket-level locking or lock-striping
CopyOnWriteArrayList creates a new copy on every write

⏱️ Performance and Big-O Complexity

Operation	Synchronized List	CopyOnWriteArrayList	ConcurrentHashMap
Read (get)	O(1) (locked)	O(1)	O(1)
Write (add/put)	O(1) (locked)	O(n) (copy)	O(1)
Iteration	O(n) (locked)	O(n) (lock-free)	O(n) (weakly consistent)

🧠 CopyOnWriteArrayList is ideal for read-heavy, write-light scenarios.

🚀 Real-World Use Cases

Use Synchronized Collections When:

You're maintaining legacy systems
You have very low thread contention
Migration to concurrent collections isn't feasible

Use Concurrent Collections When:

You expect high concurrency
You need non-blocking reads and concurrent writes
You want scalable thread-safe collections for modern apps

🆚 Comparisons and Alternatives

Feature	SynchronizedMap	ConcurrentHashMap
Thread safety	Full lock	Segment or CAS-based
Iteration safety	Must manually sync	Weakly consistent
Null keys/values allowed	Yes	No

🧠 Functional Programming Support (Java 8+)

ConcurrentHashMap<String, Integer> scores = new ConcurrentHashMap<>();

scores.put("Alice", 95);
scores.put("Bob", 85);

scores.entrySet().stream()
      .filter(entry -> entry.getValue() > 90)
      .forEach(System.out::println);

📛 Common Pitfalls and Anti-patterns

❌ Forgetting to synchronize iterations on synchronized collections
❌ Using CopyOnWriteArrayList in write-heavy systems (huge overhead)
❌ Storing nulls in ConcurrentHashMap (throws NullPointerException)
❌ Mixing non-thread-safe and thread-safe collections interchangeably

✅ Always choose collections that match your read/write ratio and concurrency demands.

🧼 Refactoring Legacy Code

Before:

Map<String, String> oldMap = Collections.synchronizedMap(new HashMap<>());

After:

Map<String, String> safeMap = new ConcurrentHashMap<>();

Improves concurrency without blocking all threads on every access.

✅ Best Practices

Prefer ConcurrentHashMap for multi-threaded maps
Use CopyOnWriteArrayList for read-heavy use cases
Avoid synchronized wrappers in modern Java codebases
Never synchronize externally on concurrent collections

📌 What's New in Java 8–21?

Java 8

ConcurrentHashMap redesigned with CAS-based operations
Introduction of forEach, computeIfAbsent, merge, etc.
Stream and lambda support for collections

Java 9

Immutable collections via List.of(), Map.of()

Java 10–17

Type inference (var) and performance tuning
Cleaner syntax for concurrent structures

Java 21

Enhanced performance under virtual threads
Works seamlessly with structured concurrency APIs

🔚 Conclusion and Key Takeaways

Synchronized collections are simpler but less performant under load.
Concurrent collections are optimized for modern hardware and workloads.
Always match the collection type with your application's concurrency profile.

❓ Expert-Level FAQ

Should I replace all synchronized collections with concurrent ones?
Not always — depends on access patterns and performance goals.
Is Collections.synchronizedList() obsolete?
Not obsolete, but less preferred than concurrent alternatives.
Why can't I store nulls in ConcurrentHashMap?
To avoid ambiguity with get() returning null for both missing and null-mapped keys.
How is iteration different in concurrent collections?
It’s weakly consistent — won't throw ConcurrentModificationException.
Are concurrent collections lock-free?
Some, like ConcurrentLinkedQueue, are fully lock-free. Others use fine-grained locking.
How do I choose between CopyOnWriteArrayList and Vector?
Always prefer CopyOnWriteArrayList for modern, read-heavy scenarios.
Do I need to externally synchronize concurrent collections?
No. It's a common anti-pattern.
Can I use concurrent collections in a single-threaded app?
Technically yes, but it adds unnecessary overhead.
Which is faster for iteration – synchronized or concurrent?
Concurrent, since it's not fully locked during iteration.
Are concurrent collections safe for all use cases?
Yes, but you still need to manage compound actions atomically when needed.