What is the difference between the HashMap and Hashtable classes in Java?

In Java, both HashMap and Hashtable are used to store and manipulate key-value pairs. However, there are some key differences between the two classes.

1. Synchronization

The most significant difference between HashMap and Hashtable is synchronization. HashMap is not synchronized, which means that it is not thread-safe. On the other hand, Hashtable is synchronized, which means that it is thread-safe. This makes Hashtable slower than HashMap for single-threaded applications.

2. Null keys and values

HashMap allows one null key and multiple null values. On the other hand, Hashtable does not allow null keys or values. If you try to add a null key or value to a Hashtable, it will throw a NullPointerException.

3. Inheritance

Hashtable is a legacy class that has been present in Java since version 1.0. On the other hand, HashMap is a newer class that was introduced in Java 1.2. HashMap is a subclass of the AbstractMap class, which means that it can be easily extended to create new map implementations. Hashtable, on the other hand, is not a subclass of the AbstractMap class.

4. Iteration

The iterators returned by the two classes are slightly different. The iterator returned by HashMap is fail-fast, which means that it will throw a ConcurrentModificationException if the map is modified while the iteration is in progress. The iterator returned by Hashtable is not fail-fast.

Conclusion

Both HashMap and Hashtable are used to store key-value pairs in Java, but they have some significant differences. HashMap is faster and allows null keys and values, while Hashtable is slower and does not allow null keys or values. HashMap is also a subclass of the AbstractMap class, which makes it more extensible than Hashtable. However, Hashtable is synchronized, which means that it is thread-safe.

The HashMap and Hashtable classes in Java are both used to store data in key-value pairs. However, there are some key differences between the two classes:

• HashMap is non-synchronized, while Hashtable is synchronized. This means that HashMap is not thread-safe, and multiple threads cannot access it concurrently without proper synchronization. Hashtable, on the other hand, is thread-safe and can be accessed by multiple threads without any additional synchronization.
• HashMap allows null keys and values, while Hashtable does not. This means that you can store a null key or value in a HashMap, but you cannot do so in a Hashtable.
• HashMap is faster than Hashtable. This is because HashMap is not synchronized, and therefore does not have to perform the additional synchronization overhead that Hashtable does.
• HashMap's iterator is fail-safe, while Hashtable's iterator is not. This means that if you iterate over the keys and values in a HashMap and then modify the HashMap while the iterator is still in use, the iterator will still return the correct values. However, if you iterate over the keys and values in a Hashtable and then modify the Hashtable while the iterator is still in use, the iterator may return incorrect values.

In general, you should use HashMap if you need a non-synchronized map that can be accessed by multiple threads. You should use Hashtable if you need a synchronized map that can be accessed by multiple threads without any additional synchronization.