What are the differences between HashMap and Hashtable?

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HashMap and Hashtable are both classes in Java used to store key-value pairs, but they have several key differences:

1. Thread Safety:

   • HashMap is not synchronized, meaning it is not thread-safe and should be used in single-threaded environments or with external synchronization for multithreading.

   • Hashtable is synchronized, meaning it is thread-safe for concurrent access but suffers from lower performance due to overhead.

2. Performance:

   • HashMap is generally faster than Hashtable because it does not have the overhead of synchronization.

   • Hashtable is slower due to synchronized methods, even if synchronization is not needed.

3. Null Keys and Values:

   • HashMap allows one null key and multiple null values.

   • Hashtable does not allow any null key or null values. Trying to insert a null key or value throws a NullPointerException.

4. Legacy:

   • Hashtable is a legacy class, part of the original Java 1.0. It is retained for compatibility but generally discouraged in favor of newer classes.

   • HashMap is part of the Java Collections Framework, introduced in Java 1.2.

5. Iterator:

   • HashMap uses Iterator, which is fail-fast and throws ConcurrentModificationException if modified while iterating.

   • Hashtable uses Enumerator, which is not fail-fast.

6. Subclassing:

   • HashMap can be subclassed more freely.

   • Hashtable has many synchronized methods, making subclassing more complex.

Conclusion: Use HashMap for non-threaded applications and Concurrent Hash Map for thread-safe scenarios instead of Hashtable.

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