HashMap vs HashTable

HashMap and HashTable are both data structures in Java used for storing key-value pairs. They offer efficient retrieval and access based on keys, but they differ in various aspects, including synchronization, null values and keys, iteration, performance, inheritance, and more.

1. Synchronization: HashTable is synchronized, which means it is thread-safe by default. Multiple threads can access a HashTable concurrently without the need for external synchronization. In contrast, HashMap is not synchronized by default. However, synchronization can be achieved by using external synchronization techniques such as the Collections.synchronizedMap() method or by using the ConcurrentHashMap class, which provides thread-safe operations with better scalability than HashTable.

2. Null Values and Keys: HashTable does not allow null keys or values. If you attempt to store a null key or value in a HashTable, a NullPointerException will be thrown. On the other hand, HashMap allows a single null key and any number of null values. This distinction can be useful when you need to store null values or handle scenarios where keys may or may not be present.

3. Iteration: HashMap offers an iterator known as a "fail-fast" iterator. If the map is structurally modified (such as adding or removing elements) while iterating, a ConcurrentModificationException will be thrown to prevent potential inconsistencies. In contrast, HashTable uses an enumerator that does not throw this exception. However, the enumerator is not fail-safe and may throw unpredictable exceptions if the underlying HashTable is modified during iteration.

4. Performance: HashMap generally offers better performance than HashTable. The synchronized nature of HashTable introduces additional overhead, making it slightly slower in single-threaded scenarios compared to HashMap. HashMap is the preferred choice for most applications unless explicit synchronization is required.

5. Inheritance: HashTable is a legacy class that has been present since earlier versions of Java, while HashMap is part of the Java Collections Framework introduced in Java 1.2. As a result, HashMap offers more flexibility, additional features, and better integration with other collection classes. It implements the Map interface and provides features such as keySet(), entrySet(), and values() methods, which are not available in HashTable.

6. Resizing and Rehashing: Both HashMap and HashTable dynamically resize and rehash their internal structures to maintain a balance between the number of elements and the capacity of the underlying array. However, the resizing mechanism of HashMap is more efficient because it uses the concept of the load factor to determine when to resize the internal array, while HashTable uses a fixed size and needs to be manually resized when the number of elements exceeds its capacity.

7. Concurrent Modification: While both HashMap and HashTable support concurrent access, they handle concurrent modifications differently. If multiple threads modify a HashTable concurrently, the behavior is well-defined and predictable due to the inherent synchronization. However, in HashMap, concurrent modifications by multiple threads can lead to inconsistent results or even data corruption, unless proper synchronization mechanisms are employed.