What is the difference between HashMap and Hashtable?

HashMap and Hashtable are two classes in Java's Collections Framework used for storing key-value pairs. While both implement the Map interface and serve a similar purpose, they possess distinct characteristics that are crucial to understand for effective application development, especially regarding thread-safety, null handling, and performance.

Core Distinctions

The main differences between HashMap and Hashtable center around their thread-safety mechanisms, how they handle null keys and values, their performance characteristics, and their historical context within the Java platform. These factors heavily influence which implementation should be chosen for a given scenario.

Detailed Comparison Points

• Synchronization: Hashtable is synchronized, meaning it is thread-safe and can be used directly in multi-threaded environments without external synchronization. HashMap is non-synchronized, making it not inherently thread-safe and suitable for single-threaded environments or when external synchronization (e.g., using Collections.synchronizedMap()) is explicitly managed.
• Null Keys and Values: HashMap allows one null key and multiple null values. Hashtable, on the other hand, does not allow any null key or null value; attempting to insert them will result in a NullPointerException.
• Performance: Due to the overhead of synchronization, Hashtable is generally slower than HashMap. HashMap offers superior performance in single-threaded scenarios where thread-safety is not a concern or is handled externally.
• Inheritance: Hashtable is a legacy class that extends Dictionary (an obsolete class) and implements the Map interface. HashMap is part of the newer Java Collections Framework, extending AbstractMap and implementing the Map interface.
• Fail-Fast Iterators: HashMap's iterators (obtained via keySet().iterator(), entrySet().iterator(), values().iterator()) are fail-fast, throwing a ConcurrentModificationException if the map is structurally modified during iteration. Hashtable's Enumeration is not fail-fast, but its Iterator (if obtained from its Map views) is fail-fast.
• Java Version: Hashtable was introduced in Java 1.0 as part of the original Java API. HashMap was introduced in Java 1.2 as part of the new Java Collections Framework.

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