Difference between HashMap and Hashtable
HashMap and Hashtable are both implementations of the Map interface in Java, which allows storing key-value pairs. However, there are several differences between them:

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1. Synchronization:
- Hashtable is synchronized, meaning it is thread-safe for use in concurrent environments. All of its methods are synchronized, making it safe for use in multi-threaded applications.
- HashMap is not synchronized by default. It is not thread-safe for concurrent use. However, you can make a HashMap synchronized using the Collections.synchronizedMap() method.
2. Performance:
- HashMap typically performs better than Hashtable in single-threaded environments due to its lack of synchronization. Since HashMap is not synchronized, it avoids the overhead of synchronization, making it faster.
- Hashtable’s synchronized nature can lead to performance overhead in multi-threaded environments, as all method calls are synchronized.
3. Null Keys and Values:
- HashMap allows one null key and any number of null values. This means that a HashMap can have at most one key with a null value.
- Hashtable does not allow null keys or values. If a null key or value is attempted to be inserted into a Hashtable, it will throw a NullPointerException.
4. Iterators:
- HashMap’s iterators are fail-fast. If the map is structurally modified at any time after the iterator is created, except through the iterator’s own remove method, the iterator will throw a ConcurrentModificationException.
- Hashtable’s iterators are not fail-fast. They do not throw ConcurrentModificationException if the map is structurally modified during iteration.
5. Legacy:
- Hashtable is a legacy class and has been present in Java since the early versions. It is part of the original Java Collections Framework.
- HashMap was introduced later and is part of the Java Collections Framework introduced in Java 1.2. It is the preferred choice over Hashtable in most scenarios.
In summary, HashMap is preferred over Hashtable in most scenarios due to its better performance, flexibility, and the ability to handle null keys and values. Hashtable’s synchronized nature makes it suitable for use in multi-threaded environments where thread safety is a requirement. However, for concurrent use cases, ConcurrentHashMap is often preferred over Hashtable due to better performance and scalability.