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

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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

  1. Should I replace all synchronized collections with concurrent ones?
    Not always — depends on access patterns and performance goals.

  2. Is Collections.synchronizedList() obsolete?
    Not obsolete, but less preferred than concurrent alternatives.

  3. Why can't I store nulls in ConcurrentHashMap?
    To avoid ambiguity with get() returning null for both missing and null-mapped keys.

  4. How is iteration different in concurrent collections?
    It’s weakly consistent — won't throw ConcurrentModificationException.

  5. Are concurrent collections lock-free?
    Some, like ConcurrentLinkedQueue, are fully lock-free. Others use fine-grained locking.

  6. How do I choose between CopyOnWriteArrayList and Vector?
    Always prefer CopyOnWriteArrayList for modern, read-heavy scenarios.

  7. Do I need to externally synchronize concurrent collections?
    No. It's a common anti-pattern.

  8. Can I use concurrent collections in a single-threaded app?
    Technically yes, but it adds unnecessary overhead.

  9. Which is faster for iteration – synchronized or concurrent?
    Concurrent, since it's not fully locked during iteration.

  10. Are concurrent collections safe for all use cases?
    Yes, but you still need to manage compound actions atomically when needed.