HashMap vs Hashtable in Java: Key Differences & When to Use
HashMap and Hashtable both store key-value pairs, but HashMap is a modern, non-synchronized collection introduced in Java 1.2, while Hashtable is the legacy synchronized counterpart from Java 1.0.
Engineers reach for Hashtable when skimming old tutorials or copy-pasting legacy code; HashMap shows up in every new microservice because interviewers insist it’s faster. The confusion? They sound identical and both map keys to values.
Key Differences
HashMap allows one null key, any null values, is unsynchronized, and offers predictable iteration via LinkedHashMap. Hashtable rejects nulls, is thread-safe with coarse synchronization, and delivers slower, legacy enumeration.
Which One Should You Choose?
Use HashMap for single-threaded or concurrent collections with explicit locks. Pick Hashtable only when maintaining vintage code that already relies on its synchronized methods—otherwise wrap a HashMap with Collections.synchronizedMap or use ConcurrentHashMap.
Examples and Daily Life
Imagine a config reader: HashMap happily stores “theme”:null, letting users reset styles. A 2005 tax applet, however, still spins Hashtable to lock records while agents file returns, because replacing it would require re-certification.
Can I replace Hashtable with HashMap safely?
Yes, if you add external synchronization or switch to ConcurrentHashMap for thread safety.
Why does Hashtable reject null keys?
Its legacy equals() and hashCode() calls crash on null, so the designers forbade them to prevent runtime exceptions.