Hash tables are a core data structure in computer science, used to create dictionaries, associative arrays, and sets. In Java, the two main hash table implementations are HashMap and HashTable. While they share similar functionality, they differ in performance, thread-safety, and flexibility.
This article highlights the key functional and performance differences between Java’s HashMap and HashTable.
Functional Differences
Thread-Safety
The primary difference lies in thread-safety. HashTable is thread-safe and uses synchronized methods to ensure only one thread modifies it at a time. In contrast, HashMap is not synchronized, allowing multiple threads to access it simultaneously, which can cause data corruption if not handled properly.
Null Keys and Values
HashTable does not allow null keys or values. Attempts to add them will throw a NullPointerException. On the other hand, HashMap supports one null key and multiple null values, offering more flexibility.
Iteration Order
The iteration order in HashMap can change as elements are added or removed. In contrast, HashTable maintains a predictable order, based on the sequence in which items were added.
Performance Differences
Synchronization Overhead
Since HashTable synchronizes every operation, it adds overhead, even when used in single-threaded environments. HashMap avoids this overhead, making it faster in such scenarios.
Hashing Efficiency
HashMap uses an improved hashing mechanism to reduce collisions and enhance performance. It also dynamically resizes to maintain a low load factor as elements grow, further reducing collisions.
Initial Capacity
HashTable starts with a default capacity of 11, while HashMap begins with 16. This allows HashMap to store more elements initially, delaying the need to resize, which can improve performance in larger datasets.
Choosing Between HashMap and HashTable
HashMap is typically faster and more versatile but lacks thread-safety. HashTable ensures thread-safety at the cost of slower performance due to synchronization. Your choice should depend on whether thread-safety is a critical requirement for your application.
More Information
- Oracle HashMap (https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html)
- Oracle HashTable (https://docs.oracle.com/javase/8/docs/api/java/util/Hashtable.html)