What are the differences between a HashMap and a Hashtable in Java?

Deciding between HashMap, Hashtable, and ConcurrentHashMap

Thread sync and performance

Non-threaded applications should use HashMap due to its high performance. For dealing with multiple threaded modifications, go for ConcurrentHashMap, a more efficient option than synchronized Hashtable. ConcurrentHashMap provides features such as atomic operations, which Hashtable lacks.

Key/value nullability

A unique trait to HashMap is its acceptance of one null key and any number of null values. In contrast, Hashtable locks the door for null entries.

Iteration behavior

A HashMap iterator is the Sherlock Holmes of iterators—it fast-fails when it detects a structural modification outside of its remove operations. In the world of Hashtable, the enumerations are more like Phileas Fogg, continuing their journey unflustered by any structural changes.

Evolution and legacy

Over time, HashMap has emerged as the modern alternative to Hashtable. For those dealing with legacy codebases, consider switching over from Hashtable to HashMap or ConcurrentHashMap for their speed and additional features.

Optimal Usage Scenarios

The Speedster (HashMap in single-threaded applications)

When you're in a race against time, and thread safety isn't on the cards, HashMap is the flash of collections you need.

The Security Savvy (Hashtable in legacy code)

When wandering grandfather's legacy codebase, the vintage Hashtable reigns supreme, firmly gripping its thread safety mantra.

The Multi-tasking Maestro (ConcurrentHashMap for concurrent modifications)

For handling multiple threads without compromising performance, look no further than the ConcurrentHashMap, equipped with modern thread-safe features.