[Pattern] Legacy synchronized collections to modern alternatives

[brunoborges]

Category

collections

Slug

legacy-synchronized-collections

Title

Legacy synchronized collections to modern alternatives

Difficulty

intermediate

Since JDK

5

Summary

Replace Vector and Hashtable with collections selected for actual mutability and concurrency needs.

Old code label

Legacy synchronized collection

Old code

Hashtable<String, Session> sessions = new Hashtable<>();
sessions.put(id, session);

Modern code label

Concurrent Collections API

Modern code

ConcurrentMap<String, Session> sessions = new ConcurrentHashMap<>();
sessions.put(id, session);

Explanation

Vector and Hashtable synchronize every operation through legacy APIs. Modern code should normally use ArrayList or HashMap for thread-confined data, and purpose-built concurrent collections when shared mutation is required.

Why the modern way wins

🎯 Intentional concurrency — The chosen type documents whether sharing is expected.
⚡ Better scalability — ConcurrentHashMap avoids a single table-wide lock for normal access.
🧩 Modern APIs — Works naturally with current collection and concurrent-map operations.